I would think that a carrier between any citypair would mean 'travel must be via' this carrier only (marketing flt no - no operating carrier).

For example, the first one:

BKK(UL)HKG-SEL-LAX(UA)SFO

Means routing is valid if UL is used between BKK-HKG, any carrier between SEL-LAX, and only UA LAX-SFO. Same thing goes if more than one carrier specified.