To construct a swept or all hex mesh on typical geometries, side faces generally have to be able to be mapped or submapped. In most cases, if manual source faces are selected with MultiZone, all other faces are treated as side faces and are mapped/submapped if possible. However, certain model characteristics can lead to problems in mapping/submapping side faces, and ultimately lead to meshing failure. You can use virtual topologies to correct most issues and obtain successful side face handling and mesh generation. In some cases, you may also want source faces to be mapped/submapped, but that should be a lower priority.
In addition, imprinted regions must have a clear path of connecting side faces and handling intersections throughout the path.
Note: If all faces of a body have a face meshing control set to Mapped, MultiZone will perform additional steps to ensure an all-mapped hex mesh is created. This can be important for cases where source faces of one body are side faces of an adjacent body so the entire body must be meshed with mapped faces in order to mesh the full model.
For guidelines that explain some common situations and steps you can take to resolve faces into mappable regions, see: