Face Pre

With Game Engine mode enabled, AdvancedSkeleton disables face setup sections that rely on non-exportable deformers.
Face Pre: Game Engine
Turn on Game Engine mode when the character face rig needs to be exported to a game engine.
If the character rig is to be used in a game engine, there are restrictions on what kinds of deformers can be used.
Most game engines only support deformation through skin clusters and blend shapes.
For a rig that will stay in Maya, many more deformers are available, such as lattice deformers, clusters, and soft mods.

What changes
Turning on the Game Engine option disables sections that use deformers which cannot be exported.
In the Face Pre interface, this affects sections such as DeltaMush, HeadSquash, and Custom Controllers.
Use Game Engine mode before building a face setup intended for game export, so unsupported deformation features are not added to the rig.