2D primitives and 2D composite regions can be used as locations to create boundary conditions and domain interfaces.

If your assembly has more than one 3D primitive and they share a common boundary, then at least one pair of 2D primitives will exist at the common boundary. One 2D primitive of each pair will bind one of the 3D primitives that shares the common boundary.

It is not possible for a region to span more than one domain in a single boundary condition.