There are two categories of answerable (mukallaf) Muslims.

1. Mujtahid (One who is qualified in Ijtihad) -

A person who so much of knowledgeable ability and understanding that he is able to understand Quranic injunctions, the purpose of the Kalaam (Divine Scripture), is able to extract rules fro it, has complete knowledge over the applicable (naasikh) and inapplicable (mansookh), exceptional linguistic command over Arabic and is well versed in all Quranic verses and Ahadith of laws. Besides these qualities, he must be intelligent and have a comprehending mind. Tasfeeraate-Ahmadia, etc.

2. Ghair-Mujtahid / Muqallid -

A person who has not reached such a level.Taqleed is necessary for a Ghair-Mujtahid and is not allowed for a Mujtahid.

There are six types of Mujtahids,

1. Mujtahid fi'sh-Shar'i -

Those scholars who make the rules (qawaaid) to be followed for ijtihaad, eg. the Four Imaams, ie. Abu Hanifa, Shafee, Maalik and Ahmad ibn Hanbal (RadiAllahu Anhum) 

2. Mujtajid fil Mazhab -

Those learned scholars who follow (taqleed) the principles introduced by the Four Imams and extract or deduce supplementary principles in the religion for the convenience of their followers; for example, Imam Abu Yusuf, Muhammad Ibne Mubarak, who are the followers of Imam Hanifa in respects of Principles and are themselves Mujtahids in masaail.

3. Mujtahid fil Masaail -

are those scholars who are Muqallids of both the qawaaid and masaail but can prove through the Quran, Hadith and other proofs, laws regarding which no elucidation by the Imaams can be found, eg. Imaam Tahaawi,

Qaadhi Khan, Shams al A'imma Sarkhasi, etc.

4. Ashaab al-Takhreej -

Those scholars who cannot make ijtihad absolutely. Howver, they can clarify in detail an abridged ruling of an Imam, eg. Imam Kharkhi.

5. Ashaab al-Tarjeeh -

Those scholars who can predonderate a particular ruling from several other narrated rulings of the Imaam regarding a single matter. Meaning, if in a certain rule there are two narrated verdicts of Imaam Abu Hanifa, these scholars can give tarjeeh of a certain one. Similarly, when there is a difference in ruling between Imaam Abu Hanifa and the Saahibain (Imaam Abu Yusuf and Imaam Muhammad), they can give tarjeeh to the ruling they deem fit, eg. the authors of Qudoori and Hidaaya.

6. Ashaab al-Tameez -

Those scholars who can differentiate between the general ruling and uncommon narrations, as well as between the weak, stronger, strongest verdict, so as to act upon the reliable one and not on unfamiliar and weak narrations of verdict, eg. the authors of Kanzad-Daqaaiq and Durre-Mukhtar, etc.

Any person who does not possess any of the above-mentioned six qualities is Muqallid-e-Mehz (one who has to make Taqlid absolutely), eg. ordinary people like you and me, the general Ulema who only to research rules in the religious books and convery them to the masses.

It is haraam for a Mujtahid to make Taqleed. Thus, the Mujtahid belonging to a particular level of the six will  not make Taqleed of those equal to his level. However, such a scholar will be a Muqallid of those Mujtahids above his rank, eg. Imam Abu Yusuf and Imam Muhammad, who are Muqallids of Imam Abu Hanifa in principles (usool) and qawaail but are not Muqallids to him in Masaail. 

With this elaboration of ours, the objections of the 'ghair muqallids get answered who often say that when Imam Abu Yusuf and Imam Muhammad are themselves the hanafis, then they are at times raising objections against Hadrat Imam Abu Hanifa. In this connection it can be said that in respect of Usul (Fundamental Principles) and (Qawa'id (rules and Regulations) they are the muqallids and as such they do not differ with the great Imam. While in respect of subsidiary and extraneous problems, they are the mujtahids and can express their independent opinions in which they are not the muqallid. The above exposition also answers the objection that at times the Hanafis adopt the verdicts of their other Imams in some matters and give fatwa by passing the statement of Hadrat Imam Abu Yusuf. In view of their behaviour why should they call themselves as Hanafis, the answer is that somes jurists (Fuqaha) are also the Ashabul Tarjih and it is their prerogative to give preference in certain problems and its acceptance cannot be taken as disobedience to the Imam. In the circumstances the followers of Hadrat Imam Abu Hanifa remain Hanafis instead of belonging to any other sub-division in matters of taqleed.

The statements of Imam Abu Yusuf and Muhammad bin Mubarak are based on the fundamental enunciations of Hadrat Imam Abu Hanifa and can therefore be said to belong to him isince in the final analysis they are based and derived from the Qu'ran and the Sunnah. The great Imam has said that anything belonging to and derived from the two sources of Islam, namely the Qur'an and the Ahadith, is part of his religion (matter of Belief or Imaan). Therefore to accept any statement from Imam Abu Yusuf or Imam Muhammad bin Mubarak is as good as taking from Hadrat Imam Abu Hanifah himself, just in the same way that to act on any Hadith is like acting upon the Qur'an itself.

Some Ghair muqallids say that they possess the capability (knowledge) of doing Ijtihad and as such they do not follow (taqlid) of anyone else. This is not more than wishful thinking. The fact is that such tall talkers do not know and cannot dream what great knowledge is essential to be mujtahid in the real sense. Hadrat Imam Razi, Imam Ghizali, Imam Tirmizi. Imam Abu Daud, Hadrat Ghaus Pak, Hadrat Bayazid Bustami, Shah Bahaul Haq Naqshbandi have been scholars of learning of such great calibre and prominence that they are rightly regarded as the pillars of Islam; yet none of them and their likes ever claimed to be the Mujtahids; they remained Muqallids and they preferred to call themselves as Hanafi, Shafa'ee, Maliki or Hanbali In their view to be a mujtahid needed the quantum of knowledge and spiritual insight before which their own knowledge from their point of view fell short of the eminence that the station of ijtihad demanded. As against them, now adays it has become a sort of fashion in certain quarters that they do not feel any shame or reluctance in branding themselves as mujtahids (!) while the fact is that they cannot even pronounce the names of the books of Ahadith correctly!!

Extracted from 'JAA’AL HAQQ WA ZAHAQAL BAATIL'