Classical analysis identifies several reasons for faṣl. The three central cases here are complete disjunction (كمال الانقطاع), complete connection (كمال الاتصال), and near-complete connection (شبه كمال الاتصال). The first makes coordination inappropriate; the second makes it superfluous; the third treats the next sentence as an answer to an implied question.
Ground for faṣlCentral idea
Complete disjunctionDifferent communicative functions
Complete connectionSecond explains or reinforces first
Near-complete connectionSecond answers an implicit question
These labels matter because they explain what meaning would change if a conjunction were inserted.