The reserved word VOID is treated syntactically the same as a parameter redefinition. If this word appears in any of the LAYER-qualified positions above, then that (region,layer) compartment will be excluded from the domain.
REGION 2 'blob' { the embedded blob }
LAYER 'Can' VOID
START 'ring' (R,0)
ARC(CENTER=0,0) ANGLE=360
The example problem "Samples | Misc | 3D_Domains | 3D_Void.pde" demonstrates this usage.