comp_propcombine_volume
comp_propcombine_volume is a internal brush entity available in 
Portal 2: Community Edition.
…
It is also available in all games with 
TeamSpen's Hammer Addons installed. It combines specified props within it volume in to a single one, thus optimizing the rendering of them.
Note:If you are experiencing problems with static prop combining not working, make sure srctools.vdf in your vmf's directory is set up with the correct path to studiomdl.exe
Keyvalues
- Name
(name)<string> - Two sets with the same name will be treated as one.
- Model Filter
(prop)<model path> - If set, a combinable model used to filter which others will be combined. The propcombine set will only apply to models which could be combined with this one.
- Skin Filter
(skin)<integer> - The skin for the Model Filter.
See also
