From the help file...
Search: /sline
mIRC Commands p to z.
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/sline [-a|r] <#channel> <N|nick>
Selects or deselects lines in a channel nickname listbox.
It can select either the Nth nickname in a listbox, or a specified
nickname.
If you do not specify any switches, any existing selections
in the listbox are cleared. If you specify the -a switch then
the specified is selected without affecting the selection
states of other lines. If you specify the -r switch then the
specified item is deselected.
