«هو علم يعرف به أحوال اللفظ العربي التي بها يطابق مقتضى الحال»
It is the discipline through which one knows those conditions and forms of Arabic expression by which it conforms to the requirement of the situation.
The forms of expression include declarative and nondeclarative forms, degrees of emphasis, arrangements of subject and predicate, and appropriate concision or elaboration. A change of expression is valuable when it answers a communicative need.
“زَيْدٌ قَائِمٌ” and “إِنَّ زَيْدًا لَقَائِمٌ” communicate the same basic predication with different strength of emphasis. Their suitability depends on the listener's state.