Assign Selected to Joint assigns the currently selected vertices and points to the currently selected bone joint. You may have more than one bone joint selected.
Select a bone joint, chose any vertex selection tool, and then use Shift-Select to select vertices without unselecting the bone joint.
Auto-Assign Selected assigns the currently selected vertices and points to the joints that MM3D thinks are most appropriate. You can increase and decrease the tendency to assign a vertex to multiple joints.
Generally auto-assign works fine when the bone joints are obvious, requires some adjustment in weight when near a joint, and doesn't do a very good job at all if the vertex is betwen several joints but not along one of the bone lines. The algorithm is not perfect, but is a good starting point to begin fine-tuning manually.
Remove All Influences from Selected will unassign any bone joints that are assigned to the selected vertices or points.
Remove Selected Joint from Influencing will remove the selected bone joint as an influence from any vertex or point.
Convert Multiple Influences to Single will find vertices and points with multiple influences and remove all the influences except for the strongest influence.
Select Joint Influences will select any bone joints that are assigned as influences to any of the selected vertices or points.
Select Influenced Vertices will select any vertices that are influenced by the selected bone joint.
Select Influenced Points will select any points that are influenced by the selected bone joint.
Select Unassigned Vertices will select any vertices that are not influenced any bone joints.
Select Unassigned Points will select any points that are not influenced any bone joints.