Patchmesh Editing
Patchmesh geometry works differently than brush geometry in a number of ways. They have dynamic level of detail (LoD) so the polycount of its shape is determined at runtime. In the editor, you define the patch's control points and the actual surface is interpolated between those control points.
The purpose of this is to enable varying levels of LoD optimizations, so that curved surfaces can be rendered with a low polygon shape at a distance, and a high polygon shape when up close. In addition, because they lack a fixed structure, patches are ignored by BSP and VIS. The dynamic LoD nature of patches is clearly a performance benefit, but due to their unique properities, you need to be careful using them to prevent graphical artifacts and unintended negative performance issues.