One of the neat things about ON hooks is that multiple hooks of the same type may be given different priorities. A serial number may be any number, positive or negative; zero is the default and should not be used directly. The numbers themselves don't really matter, rather their relation to other serial numbers is what's important. For example:
on hook "*" echo foobar on #-hook -5 "*" echo booya on #-hook 5 "*" echo yadda yadda yadda
Then, when any HOOK command is issued, the following three lines will be displayed, in this order:
booya foobar yadda yadda yadda
In general, it is always advisable to use serial numbers, if for no other reason than to prevent hooks from conflicting with each other. This is particularly true of the TIMER hook, which should always use serial numbers.