No, you should read the help file on $did().sel (and in fact the entire page on dialogs). Also, when you don't know what something does exactly, USE /echo. Bold cannot stress this enough, /echo is the best way to figure out how stuff works.
$did(name,id,0) -> total number of selected lines
$did(name,id,1) -> index of 1st selected line
$did(name,id,2) -> index of 2nd selected line
Inside an on DIALOG event $did(name,id,N) can be $did(id,N)
