trigger_changelevel has two added features in Quoth. If “startdisabled” is set to 1, the changelevel won’t be active until triggered via its targetname. Once activated it must then be touched to work as normal.
This simply allows for the player to exit through the same location they entered the map.
The “mapindex” key is used with the Session Mapindex feature in Quoth. The Session Mapindex is a value which is preserved between maps to track which maps the player has visited, and is read by the info_mapgate and info_endgate entities. trigger_changelevel is the way to modify the Session Mapindex; when you leave a map through a trigger_changelevel, the value in the mapindex key is added to the Session Mapindex.