1 for vertices
2 for edges
3 for borders (UV seams, not an easy way without scripting now)
4 for polys
5 for elements (UV islands, now a separate toggle)
Bonus points if the subobject icons behave the same way as in Edit(able) Poly, too, i.e. Ctrl+clicking to convert one selection type to another, Shift+clicking to convert to selection border and Ctrl+Shift+clicking to convert convert selection excluding selection border.