Func instance

It inserts another VMF into the current map. Similar to C++ member functions, a  can be given parameters that change what it does, and the original instance VMF can be edited at any time and instantly apply to all other VMFs that use it. This saves mappers the tedious work of updating them all individually (though, maps already using the instance will have to be recompiled). Instances are used very extensively in Portal 2 for testing elements.

When the map is compiled, all instances are collapsed into the main map, effectively copied and pasted in with the correct positions and properties. In the engine, instances technically don't exist.