XF's meshing algorithm utilizes dielectric volume averaging (DVA) to consider the volume around each cell edge and set material properties based on the average of those findings. This affects the mesh of a part's surface where it meets other materials rather than the internal, homogeneous area of the part.
The finite-difference time-domain (FDTD) method discretizes CAD geometry into cell edges. It is common to think of discretization with regard to cells, but users should take care not to let this common association abstract their understanding of the FDTD update equations. Each cell is associated with an electric field but it is the cell's edges, not the cell itself, that carry the electromagnetic information during timestepping. Therefore, each cell edge represents the volume around it, which is roughly one third the size of the cell.