How does OffLatticeBoundaryCondition3D apply boundary information on the surface mesh to lattice base?

Hello everyone, I am working on an aortic flow case by Palabos, and want to know the flow rate of each opening. However, not all the openings are parallel or perpendicular to the coordinate system. So, I cannot use Box3D to circle an exact domain, which may bring error to the results.

Then, I noticed the class - OffLatticeBoundaryCondition3D seems to combine model object (include boundary geometry, boundary profile information, and other surface information) with voxelizedDomain and lattice which are body base instead of surface geometry. What I feel is in OffLatticeBoundaryCondition3D class, there should be some operations applying information on surface geometry (boundary, boundaryProfiles) to all the elements, which are intersected with the surface, and make them as the corresponding boundary condition on the lattice base.

So, my question is: did I guess right? If yes, could anyone tell me or is there any resource talking about how to implement this process (maybe I can find some hints to solve my problem, just like the boundary nodes are marked as some certain number so I can distinguish boundary nodes from fluid nodes)? If not, how does the Palabos apply the boundary condition on the lattice base (because finally the program is iterating the lattice base instead of surface geometry)? Or, is there an internal function to get the flowrate for each opening but I just didn’t find that?

Thank you all in advance.