AFAIK, rule of convention and common sense.

Once you get away from the first four frets of the fretboardboard where the nut acts as the barre, for chords, you''ll be fingering barre chords which follow the same core logic. Any which are a partial barre which might look like an open chord further up the neck e.g. 3rd fret form D7, leaving open strings you don't strike will follow the same convention treating the third fret as the first subsequently following sequential placement convention.

Scales playing individual notes begin with the 1st or forefinger on the lowest root note. Hypothetically the same convention is applied over the respective frets of that scale, although Justin Sandercoe advocates abandoning the pinky substituting fingers 1, 2 & 3 only on the GBE strings working the pentatonic scales his reasoning being pragmatic. i.e. fingers two and three facilitate bending better.