What are the characteristics of 'illal (criteria) that distinguish them from hikam (benefits)?

This [question](https://islam.stackexchange.com/questions/46494/ "Is Hijab an outdated or an old age remedy to keep the society away from a lot of sins?") raised the point that it is hard to distinguish the term _'illah_ (criterion, Arabic: عله) from _hikmah_ (benefit, Arabic: حكمه) when it comes to *ahkām al-fiqh* (rulings of jurisprudence, Arabic: أحكام الفقه) which may cause misunderstandings of the rulings and their applications. **Question:** What are the characteristics of _'illah_ and what is its governance? The governance of _'illah_ is a bonus part. By governance of _'illah_, I am referring to the principles of jurisprudence that provide the applicability of the ruling (the ruling is tied to the criterion's presence and absence, Arabic: الحكم يدور مع علته وجودا وعدما), the interpretation of its wording (what counts is the general meaning of the words, not the specific reason for revelation, Arabic: العبرة بعموم اللفظ لا بخصوص السبب), and accepted deduction methodologies (setting criteria based on perceived benefit or through qiyās for permissibility, Arabic: التعليل بالحكمه او بالقياس فى الرخص), as examples. An answer using a single school of jurisprudence is sufficient.