sampledTriSurfaceMesh reading surfaces from region directory
IOobject ( dict.lookup("surface"), mesh.time().constant(), // instance "triSurface", // local mesh, // registry IOobject::MUST_READ, IOobject::NO_WRITE, false ),
So surfaces are taken from the mesh registry, which means
regionName/constant/triSurface for etc.
Could, however, have them taken from
mesh.time() so that they use the main