Hukm shar’ai is the speech of the Legislator related to the actions of the servants ('ibaad). It is either conclusively proven (qat'iy uthuboot), such as the Qur'an and hadith mutawatir or inconclusively proven (dhanniy uthuboot) such as the non-mutawatir hadith. If it is qat'iy uthuboot and its meaning is definitive (qat'iy udalalah), the hukm will be conclusive. An example of this is the number of all prescribed raka'ah in salat, as they are mentioned in the hadith mutawatir. Likewise, the prohibition of riba, the amputation of the hand of the thief, and the lashing of the zani (adulterer) are conclusive rules whose correctness is definite, where there is only one single conclusively proven opinion.
If the speech of the Legislator is qat'iy uthuboot and does not yield a definite meaning (dhanniy udalalah), then the included hukm is inconclusive. For example, the ayah related to jizyah in the Qur'an. The ayah is qat'iy uthuboot but the meaning is not definite. The Hanafi school stipulated that it be called jizyah and those who are required to pay it must be in a state of humiliation when rendering payment. However, the Shafi'i school did not stipulate calling it jizyah and permitted jizyah to be called double zakah They did not require for the one paying it to be humiliated, but it is enough to submit to the Islamic rules.
If the speech of the Legislator is dhanniy uthuboot, such as the non-mutawatir hadith, then the hukm included will not be conclusive, regardless of whether the meaning is qat'iy udalalah or not. For example, fasting six days in Shawwal or the prohibition of leasing agricultural land, both of which are proven through Sunnah.
The hukm Shara'i is understood from the speech of the Legislator (Khitab Ashari') through a correct ijtihad. Thus, the ijtihad of a mujtahid produces the Hukm Shara'i. Accordingly, Allah (swt)'s hukm for every mujtahid is the hukm that mujtahid arrived at through his ijtihad and what he most likely thinks to be correct.
It has been agreed upon amongst scholars that if a mukallaf (one under legal responsibility) fulfils the capacity of ijtihad in one question or more and makes ijtihad and reaches thereupon a hukm, he is not allowed to follow other mujtahideen in this issue, because it would be a taqleed of an opinion which is different to what is most likely correct in his opinion. The muttab'i is a person who has acquired some important knowledge in ijtihad and consequently follows the hukm after understanding its daleel. Accordingly, Allah's (swt) hukm for this muttab'i is the opinion of the mujtahid whom he follows. The 'aammi is the person who does not possess some important knowledge in ijtihad and hence follows the mujtahid without having knowledge of the daleel for the hukm. This 'aammi has to follow the opinion of the mujtahideen and apply the aHkam they have deduced. The Hukm Shara'i on his part is the one deduced by the mujtahid whom he follows. Therefore, the Hukm Shara'i is the hukm deduced by the mujtahid who is qualified to practise ijtihad. It is Allah's (swt) hukm for him, and he is not allowed to leave it to follow another opinion. It is also Allah's (swt) hukm for those who follow the mujtahid and they are not permitted to leave it.
If the muqallid follows a mujtahid in a hukm of any issue and acts accordingly, he is not allowed to leave that hukm for another mujtahid at all. However, it is permissible for the muqallid to follow the mujtahid in other issues, because ijma'a as-Sahabah permitted that a muqallid may ask a different 'aalim (scholar) on a different issue. If the muqallid assigned a certain School of Thought (madhab), such as the Shafi'i and committed himself to follow the entire madhab, then the following applies upon him: The muqallid is not allowed to follow any other mujtahid on an issue he has already practised according to the madhab he is following. Regarding the issues that he has not practised yet, he is allowed to follow the other mujtahideen. However, if a mujtahid reached a hukm on an issue through his ijtihad, he is allowed to abandon the result of his ijtihad and follow another opinion, if it means the unification of all Muslims on one opinion, as happened at the bai’ah of 'Uthman.