Shubhat al-dalīl involves uncertainty traceable to a recognizable scriptural or juristic indication. It differs from an unsupported claim that something simply seemed permissible.
The legal uncertainty is associated with an identifiable evidentiary basis, even if fuller analysis ultimately establishes a different ruling.
For any category, three distinct questions are useful: What gives rise to the uncertainty? What legally supports treating it as such? And what consequence does it have in the particular rule being discussed?