From Valve Developer Community
When a map is compiled VBSP converts brush faces that touch a visleaf to groups of polygons. The resulting 'brush models' are stored within the BSP file and can be claimed by entities (e.g. the world, or your brush entity). The original brushes are retained in the BSP, though the benefits of this are not clear.
In comparison to models, brushes are:
- For each individual brush model, only faces that touch a visleaf are compiled. Some of the results of this process can be seen below:
- If some parts of a brush face touch visleaves and others do not, the face will be chopped up to remove the hidden area.
There are two exceptions to these rules:
- Although they are discrete entities in Hammer, func_details are all merged into one during compile. You do not need to nodraw hidden 'detail-to-detail' faces (but you do need to nodraw world-to-detail and detail-to-world).
- Faces with translucent materials applied are never chopped. Don't apply them to internal or hidden surfaces unless you really mean to.
VBSP enforces these limits:
- 8192 brushes
- 128 faces on a single brush
- 65536 faces overall
Invisible brushes like triggers and even hint brushes count toward the limit. Faces like NODRAW do not count toward the limit. Faces textured with nodraw are removed during compile so ONLY it's collisions, boundings, and place in the world exist.