Some concepts
Patch
A patch is a named collection of boundary faces (the outer skin of your mesh). Each patch appears in constant/polyMesh/boundary and carries a type (e.g. patch, wall, empty, wedge, etc.) and is where you apply boundary conditions in 0/U, 0/p, etc.
Boundary face
Boundary face = a face that has one owner cell (the other side is “outside” the mesh/domain).
Internal face
Internal face = a face shared by two cells (it has an owner and a neighbour). Internal faces are not part of any patch.