The word ʿilm can describe a cultivated ability that enables someone to recognize particular cases, as well as established principles and rules that explain those cases.
As an ability, bayān enables a competent speaker to recognize whether a meaning is conveyed directly or inferred through a connected expression. As a body of rules, it provides the terminology and reasoning for explaining that difference.
For example, recognizing generosity as the common target of “Zayd is generous” and “Zayd is open-handed” is an individual act of understanding. Explaining how the second expression indirectly suggests the quality applies the discipline's general principles.