According to the jurist this term means that if a person is the follower of a particular school it is obligatory for him to act according to the rules laid down by that school, but those rules are not binding on the followers of other schools.
Useful information: An examples of the rule of 'Ilzam':
It is obligatory amongst the Sunnis that nikah (marriage) ceremony should take place in the presence of two witnesses. This is not, however, necessary amongst the Shi'ah. Hence, if a Sunni contracts a marriage in the absence of two witnesses his marriage, according to his belief, will be invalid and a Shi'ah may contract marriage with such a woman.