A func_group is made into solid world brushes upon map compilation. It only exists to be able to group objects in old versions of the Hammer editor, when it was still called Worldcraft. Still, you can use func_group for Vluzacn's compile operations like editing cliphull but it's highly recommended to use func_detail for this.
Priority when faces overlap zhlt_coplanarpriority : Prevents z-fighting. When two brushes have overlapping faces, this value controls which of those brushes faces have rendering priority.
Disable clipping zhlt_noclip : Enable or disable collision of entity.