A secondary sense may arise from resemblance, but in legal speech the link can involve an operative cause and its ruling. An ‘illah is a legally relevant attribute or basis tied to a consequence. Mentioning one may, in a suitable construction, allude to the other.
«الاتصال بين العلة والحكم»
A relationship between the legal cause and its ruling.
The text illustrates a relation involving purchase and ownership: a purchase contract can bring ownership about. In particular legal statements, one may speak in terms of purchase while intending ownership, or the reverse. This is context-sensitive, not an automatic equation in all circumstances.