From Valve Developer Community
env_cubemap is an internal point entity available in all Source games. It specifies a location for which a cubemap will be generated when the buildcubemaps console command is executed.
Note:This is an internal entity. When the map is compiled by VBSP it is processed and then removed: it does not exist when the map is running.
- Cubemap Size
- The resolution of each face of the cubemap. Remember that the actual number of pixels stored will be your selection times four!
- Default (usually 32x32, depending on the game)
Comparison of reflection quality (1x1 is not included because it does not reflect at all):
- Brush faces
- An optional override for individual brush faces, forcing them to use this cubemap instead of one closest to them.
To select faces, press the "Pick" button then click on them in the 3D view. Hold to toggle a face on or off.