Asking about Dolfin convert Facet region dan Physical region

Hello I’m trying to make a model that uses the dolfin program. and I have a domain/mesh file that has a .14 format and I manually convert it to xml format using a program that I make so the program can read my domain file.

But it turns out when you convert a file using dolfin convert it is converted into 3 files: The domain file, facet region file, and physical region file.

Can somebody tell me the logic behind facet region file and physical region file so maybe i can manually make the file with a program? Here i attached an example of facet region and physical region file.

I would be very grateful if I got the answer to my question.
Thank you for the attention.

It is not adviced to use dolfin-convert and xml any longer, i would suggest using meshio. See for instance: Extracting mesh regions for gmsh mesh - #16 by dokken