The following is from the translation of the book, Al-Waadih Fee Usool ul-Fiqh (The Clear Principles of Islamic Jurisprudence) by Muhammad Hussein Abdullah:
The Waajib is divided or categorised in accordance to different considerations. As such, there is a division in respect to its performance (Adaa’), another in respect to its evaluation (Taqdeer), a third in respect to its designation and a fourth in respect to the one who has been charged with it (Mukallaf).
The categories of the Waajib in consideration of its time of performance or undertaking
Waajib Mutlaq and Waajib Muqayyad (The unrestricted obligation and the restricted obligation):
1 – Al-Waajib Al-Mutlaq (unrestricted obligation):
This is what the Shaari’ (Legislator) has requested to be done without restricting it to a specific time and an example of this type is the Qadaa’ of Ramadhaan for the one who did not fast due to a Shar’iy ‘Udhr (excuse/reason) like sickness or travelling. That is because he can make Qadaa’ for it when he wishes without it being restricted to a specified year. This is the opinion of some of the Fuqahaa’ in thisissue including the Hanafiyah. Others however have stipulated that it is a condition for the person to make up (Qadaa’) for what he missed before the coming of the Ramadhaan that follows the Ramadhaan in which he didn’t fast, if the Mukallaf is capable of doing that (i.e. within a year).
Other examples include the Kaffaaraat (expiations) and unrestricted vows (Nudhoor). So whoever breaks one’s oath or vow, without being restricted to a specific time, can undertake the required expiation (Kaffaarah) directly after his breaking and undertake his vow immediately if he wishes to or he can delay it until a time of his choosing.
2 – Al-Waajib Al-Muqayyad (restricted obligation):
This is what the Shaari’ has requested to be undertaken and specified a specific and defined time for its performance or undertaking. Examples of this include the five obligatory prayers and the fasting of Ramadhaan because it is not permitted to undertake them before their set and specified time and the Mukallaf is sinful if he delays the action from its time without a legitimate legal excuse.
The Waajib Al-Muqayyad is divided into two categories: Waajib Muwassa’ and Waajib Mudayyaq.
A – Al-Waajib Al-Muwassa’: That which the time specified for its performance provides some time, room or expansiveness for it and other actions of its type to be performed. That is like the Zhohr prayer for example.
B – Al-Waajib Al-Mudayyaq (narrow): That which its specified time for its performance does not provide room for others of its type (to be performed) and this is like the fasting of Ramadhaan. That is because its time does not provide room except for the fulfilment of the obligatory fasting within it.
Aqsaam (categories of) Al-Waajib in regards to the consideration of the specification of what is required and its non-specification.
1 – Al-Waajib Al-Mu’ayyan (specified):
This is what the Shaari’ has requested in itself without providing the choice to the Mukallaf to choose between different matters (or actions). The Salaah is an example of such an obligation. As such the (obligation of the) Salaah does not fall from the Mukallaf unless he has performed it in itself, so it cannot be substituted by the recitation of the Qur’aan and fasting.
2 – Al-Waajib Ghair ul-Mu’ayyan (non-specified) or Al-Waajib Al-Mukhayyar (of choice):
This is what the Shaari’ (Legislator) has requested to be undertaken; not in itself (specifically) but within matters which have been made known. It is up to the Mukallaf to choose one from them in order to perform and fulfil such a Waajib. This Waajib could be one of two like in the Qawl of Allah Ta’Aalaa:
فَإِذَا لَقِيتُمُ الَّذِينَ كَفَرُوا فَضَرْبَ ال رقَابِ حَتَّىَٰ إِذَا أَثْخَنتُمُوهُمْ فَشُدُّوا
الْوَثَاقَ فَإِمَّا مَنًّا بَعْدُ وَإِمَّا فِدَاءً حَتَّىَٰ تَضَعَ الْحَرْبُ أَوْزَارَهَا
So, when you meet those who disbelieve strike at their necks till when you have killed and wounded many of them, then bind a bond firmly (i.e. take them as captives). Thereafter (is the time) either for generosity (i.e. free them without ransom), or ransom until the war lays down its burden (Muhammad 4).
This informs us that it is up to the Khalifah either to let the captives of war free or to exchange them by ransom.
The non-specified Waajib could also be one from three matters and its example is the Kaffaarah (expiation) of the Yameen (oath). It is obligatory upon the one who breaks the oath to do one of three things: The feeding of ten poor, or providing clothing to them or to free a slave. This is in the case when the capability exists but if the capability is not present then the obligation is Mu’ayyan (specified) and that is the fasting of three days.
The Aqsaam (categories) of the Waajib in relation to the consideration of who is Mukallaf (i.e. who has been charged to undertake the obligation)
1 – Waajib ‘Ainiy or “Fard ‘Ain”
This is what the Shaari’ has requested to be done from every Mukallaf and it is not sufficient for some to do it whilst others do not. Examples of this include the Salaah, the Sawm, being faithful to contracts and the Zakaah. The one who leaves the action is therefore sinful and another person’s undertaking of it does not remove that sin from him.
2 – Waajib ‘Alaa Al-Kifaayah:
This is what the Shaari’ (Legislator) has requested to be undertaken by a Jamaa’ah (collective) of the Mukallafeen (and not all of them). If or when some have undertaken it, then it (the obligation) falls from the rest and there is no sin upon them. Rather, all are sinful if the Waajib upon the Kifaayah (sufficiency) is not met. Examples of this include Al-Jihaad, judiciary, carrying the Islamic Da’wah to establish the Islamic State, generating industries and sciences or areas of knowledge that the Ummah is in need of, preparing the necessary force to strike fear into the enemy and praying over the dead and burying them.
The Aqsaam (categories) of the Waajib in respect to the consideration of their evaluation or non-evaluation in respect to amount or quantity
1 – Waajib Muhaddad Al-Miqdaar (the obligation where its quantity or amount is defined):
This refers to the Shaari’ having specified a defined amount or quantity related to the undertaking of the Waajib like the Zakaat and the number of Raka’aat in the obligatory prayers. The responsibility of the Mukallaf is not freed in respect to this Waajib except by undertaking and performing it upon the form that the Shaari’ has defined and specified and by the quantity or amount that has been specified for it.
2 – Waajib Ghair Muhaddad Al-Miqdaar (the obligation the amount or quantity of which is not specified):
This refers to what the Shaari’ (Legislator) has not defined and specified its amount. This is like spending in the way of Allah or the obligatory Nafaqah (spending) upon the wife and the children. That is because the Shaari’ left the specification of its amount to what the spender is capable of and in accordance to what is customary amongst the people.
Maa Laa Yatimm ul-Waajib Illaa Bihi Fa Huwa Waajib (That which the Waajib is not completed except with it, is Waajib)
The Waajib is that which the Shaari’ has requested to be undertaken by way of a decisive request (Talab Jaazim) and it can be the case that the completion of the Waajib cannot be fulfilled unless another action is undertaken upon which the fulfilment of the Waajib rests. That is like washing the elbows in the act of Wudoo’ and that is because the completion of the undertaking of the Waajib, which is washing the hands until the elbows, cannot be fulfilled unless a part of the elbows are washed. That is because the end point of what is being sought, which is the elbows, is included with the sought after matter, which is the hands/forearms. Therefore, the fulfilment of this Waajib to wash the hands/forearms rests upon the washing taking place upon a part of the endpoint i.e. washing a part of the elbows. Consequently, washing part of the elbows is Waajib because the Waajib of washing the hands (i.e. arms) is not completed except through this action i.e. washing a part of the elbows. This is built upon the Qaa’idah (principle):
“That which the Waajib is not completed except with it, is Waajib”
Similar to that is the establishment of the Khalifah or accounting the ruler because these are both Waajib as stated by the Shaari’ in the Aayaat and Ahaadeeth and the Waajib is not completed except by the establishment of a political structure that works to establish the Khalifah and account the ruler. That is because the individual cannot undertake that action because of his incapability to fulfil the undertaking of this action (and what it demands) by himself. As such establishing a political structured bloc is an obligation upon the Muslims because: “That which the Waajib is not completed except with it, is Waajib”. Consequently, if the Muslims did not establish a Takattul (structured grouping) they would be sinful because they would not have undertaken that which is necessary for them to undertake and fulfil the Waajib.
Similarly, in this way, every matter that the Waajib cannot be fulfilled except with it, is Waajib. That is if the undertaking of this matter is within the capability of the Mukallaf and if it is not within his capability then it is not Waajib. That is due to the Qawl of Allah Ta’Aalaa:
لَ يُكَلِ فُ اللَّه نَفْسًا إِلَّ وُسْعَهَا
Allah does not burden a person except in accordance to its capability (Al-Baqarah 286).