A 'boss' generally means an 'ultra' enemy, you can identify them by having a diamond icon on their health bar. Enemies with a triangle icon are just 'majors'. I think the mods have been disabled now but there used to be 'boss resist' and 'major resist', which would work on one kind but not the other. Another difference was for people working on the steps of a Black Armory weapon frame - major enemies dropped 2 radiant seeds, ultras dropped 10. Lost Sector main enemies tend to be Majors. Mars Lost Sectors counted as 'boss' kills because they were in fact ultras (and therefore were good for radiant seeds), but Subterranean Mind on Mercury was just a Major. For Mercury boss kills you had to either do the strikes or wait for the Gate Lord in the public event. For Crow's weekly bounties, the intent is to kill the Wrathborn bosses. These count as two Ultras per hunt (the one you chase, and the one that spawns after) so can complete one bounty in two hunts.