From Valve Developer Community
A prefab located in the Prefabs folder, containing a complete environment for the developer to test things out.
This prefab contains the following:
- A leak free box, using dev and
tools/toolsskyboxtextures, creating a simple 2D skybox.
- An info_player_start entity.
- A trigger_once brush without any outputs.
- A func_detail brush covered in the brush texture, marking the trigger_once brush in-game.
- A light_environment entity, lighting up the environment.
A developer could rig the trigger_once brush to any possible effect that would need testing.