Often times, they are interchangeable, as they're treated no different from the OG universe they spawned from, having their own space-time and all, even if they're linked through the stream of time itself. In essence though, it's a case-by-case basis. Taking DB for example, alternate timelines, due to the way they're set up, are in fact doubling as alternate universes unto themselves, and should be treated like such. Meanwhile, name any series here with time travel, where destroying the universe in the past, or something within it, doesn't cause a split in timelines where the present is still fine and untouched, but actively affects it, as the past and present are fully linked at the hip, in that sense? Then no, they are different timelines, as in, the original path of the same universe, has been altered into something else. They didn't fork into two separate realities, the original was erased, in place of the new one, because timeline fuckery. Does that make sense?