I was merely curious about video game compilation packs, which are common in these and have a list of games in one volume. Most of them are arcade games, but there are some from previous consoles. I had to consult the system's  speed run page to determine where  to list  a speed run for one of  these. It's not clear if the video game compilation for the system or the original system is under EMU.

Generally, an official emulator rerelease is labeled by the specific platform, and the EMU tag is for unofficial emulators. For example, Wii Virtual Console games use the platform WiiVC with no EMU tag.

I would always default to this identification method unless the leaderboard mods specifically say otherwise.

So, a game on a compilation for a console, e.g., arcade compilation for PS3. The PS3 is label rather than arcade EMU.

Generally it depends on the game. Some leaderboards accept runs from both compilations and the base game. Some leaderboard don't accept runs from the compilation since they differ so much. Sometimes there's a separate board for the compilation including a game and it is also accepted on that game's boards.

If you're just asking what you should list the platform as, list it as what you played it on. So if the compilation came out on steam for a game that was on like the ps2 or something, you list your platform as PC if you can. If not, ask the leaderboard for that game on their forums.