Speedruns submitted to the leaderboards are always done in one sitting (single-segment) unless the leaderboard rules for a specific game/category explicitly say otherwise, which is very rare these days outside of very long (24+ hour) runs. Runs done in multiple segments (segmented runs) are a thing for normal length runs, but they typically aren't given leaderboards since they're usually more of a "proof of concept" for what's possible in the game rather than something people compete with. Submitting a segmented run to a single-segment leaderboard is a form of cheating.
Not sure what gave you the impression that L4D2 is different, that game's leaderboard is also exclusively single-segment as far as I can tell.
If that's the case, each Level in that section of the leaderboard is considered a distinct run, so you do not necessarily need to do multiple Level runs in one sitting to submit for a single level. In the context of Level runs, single-segment simply means completing that level in one sitting. Some games are weird though, so always double check the rules for any games you plan on running.
