How to understand the region of Geant4

Geant4 Version:11.1.2
Operating System:ubuntu 22.04
Compiler/Version:gcc 11.4.0
CMake Version:3.22.1


If the geometry level is as below:

I want to set C to a special region. Do i set the A to a region first or set C to a region directly?

Thank you!

You can assign C to a region directly.

Following is the description in the documentation of GAMOS:
Also do not forget that regions in Geant4 have to be set in a hierarchical way: if you place a volume A in the world and inside it you place a volume B, you cannot create a new region for B unless you have explicitly created a region for A.
I cannot understand it!
Thank you!

So by default the World Region exists and contains all of the volumes you place in the geometry. In our own simulation, we define a single “InnerRegion”, which covers just our detectors, not the big cryostat or shielding outside of them. I can’t speak to the GAMOS documentation; you may want to contact them directly.