I've always been making popups using the $?= since when using $input; if you press "no" it will show "wrong syntax" or something from ChanServ.
But then by mistake I made $$input with 2 $$, and it halted if no input was entered I just thought someone might find this usefull...

Same will happen if you use $$?="Whatever"

When you have $$, it makes it a required entry, which can be useful if you have a kick w/reason, and the $? is $$?, because if you start to type a reason but then change your mind, you cancel out of it and they aren't kicked.

/!help $$ will give you more info on it.