Smart region guess what you want to select by one command.
If you call this command multiple times at the same position, it expands selected region (it calls er/expand-region).
Else, if you move from the mark and call this command, it select the region rectangular (it call rectangle-mark-mode).
Else, if you move from the mark and call this command at same column as mark, it add cursor to each line (it call mc/edit-lines).
This basic concept is from sense-region.