(note: references to specific FAA documents or cases are always appreciated!)
This is a general question and might best be considered as open-ended.
It is, however, posed for clarification of a few specific regulations:
91.205(c)(4) "if the aircraft is operated for hire, one electric landing light..."
91.409(b) "...no person may operate an aircraft carrying any person for hire .... unless within the preceding 100 hours..."
91.205(b)(12) "if the aircraft is operated for hire over water and beyond vliding distance from shore, approved flotation devices..."
There may be others regulations that reference the term in Pt 91.
The point is: do these regulations imply that neither landing lights, life jackets, nor any current 100 hr inspection are required for e.g. (1) a solo rental, (2) a rental with $\leq$ pro-rata contributing friends, (3) a checkride ? It is my understanding that none of these are not "for hire" flights but perhaps there is clarification otherwise.
i.e.1) what flight scenarios/activities are conducted under for hire circumstances and therefore must comply with the above regs?
i.e. 2) Does the term for hire describe the airplane or the pilot?