You mean you have an interface in the domain? The normals are defined even for
internal facets, but the problem is, that each facet has two possible normals
in opposite directions. To choose a particular direction, you need to set a
parent region of the interface (facet) region, as in  - there, the normals
of Gamma_12_1 are w.r.t. Omega_1, and the normals of Gamma_12_2 are w.r.t. Omega_2.
Does that help?
> Best regards,