A playlist on Launchbox is like a playlist on anything else you may use. The playlist can be unique to whatever you want it to be. It is your choice. You can have it auto-populate games or set it to manual where it only updates when you say "add this game to this specific playlist".
You can call a playlist whatever you want, have it show up under any Category, Platform, or show up on its own as a main category if you so choose.
You can see in the image I set this specific playlists up to only show up under the Category of Playlist. If I wanted I can have a playlist show up under a platform just by changing the tick boxes in the parents tab.