An entity is an entity that exists in-game.
Also, I think the light-entities are used for model lighting, for items, monsters and other stuff that uses models.
func_detail is treated like an entity in valve hammer/world craft but not by the goldsrc engine, so it does not count as model. It just a way to say the engine to don't cut the adjacent world solids.All articles on the VCD applies to Source.
https://developer.valvesoftware.com/wiki/Fu
nc_detail
func_wall is an entity and it also does not cut the world brushes because of its entity feature.
No entity will seal a level. Only world brushes will do that.And yet, someone said it doesn't count towards entity limit:
It doesn't clip surrounding brushes, doesn't count towards the entity limit (func_wall does) and still casts shadows (func_wall doesn't).And since it only exist with ZHLT or any tools built on that technology (VHLT), it doesn't exist as an entity in-game.