Hi. I have used the package a bit, and though I find powerful and convenient, it certainly lacks a bit of documentation. Nevertheless, I found out that for the simple cartesian grid that you are using, the boundaries are numbered:
- 1, 2, 3, 4: the four points around your domain (I think that it begins lower left, then lower right, upper left, upper right)
- 5: lower segment
- 6 upper segment
- 7: left segment
- 8: right segment
- and 9 is for the domain itself as far as I remember.
That does not apply in 3D obviously.
You can see an application of that in example 8 Incompressible Navier-Stokes · Gridap tutorials where the upper part of the domain has a different boundary condition (so number 6). Or on 7 Darcy equation (with RT) · Gridap tutorials where a Neumann BC is applied on segment 8 (right hand side of the domain)