The -a switch for all commands uses the window active at the time the command is issued.
umm... I still do not completely understand the -a switch. consider this code (clean install with mirc.ini in the mirc-dir, mirc 6.31, vista):
on *:ACTIVE:*: {
echo -a Active: $active
background -a image.gif
}
now I create two custom windows and... nothing happens. whenever I click into the windows, the echo would show me the correct active window where I klicked into, but the background wuld not change.
with "background $active image.gif" it works.