The help file says:
Which is very unclear imo since you would expect $input to make the dialog the active window anyway.
'activate' here means that the dialog will be displayed even if mIRC isn't the active application.
Maybe saying:
a - activate the dialog even if mIRC isn't the active application
I also would like to report that using the 'a' parameter with mIRC minimized will flash the window, if mIRC is in the tray, no flash is perfomed