+If <em>3D extrusion</em> algorithm is assigned to a sub-mesh in a mesh
+with multiple sub-meshes, the described above approach may not work as
+expected. For example the bottom face may be meshed by other algorithm
+before <em>3D extrusion</em> have a chance to project a mesh from the
+base face. This thing can happen with vertical edges as well. All
+these can lead to either a meshing failure or to an incorrect meshing.
+
+In such a case, it's necessary to explicitly define algorithms
+that <em>3D extrusion</em> implicitly applies in a simple case:
+- assign \ref projection_1D2D algorithm to the top face and
+- assign a 1D algorithm to a group of all vertical edges.
+