The numbers are for making in-game Hotkeys to use the abilities as part of Social commands you can create.
You know how you can make socials for pulling and casting spells?
There're commands to use Actions, Combat Abilities, Melody, Bandolier, Potion belt slots, AA and probably other stuff.
In your Hotkey setup box the line to activate an AA would be (I think) /altactivate #
Druid get an AA called Spirit of the Wood, which is a group HoT + DS. Often on my druid I like to announce to the group that a group HoT is coming, and having the group message in the same hotkey as the AA trigger saves a lot of time.
Rogues get an AA called Escape which ditches all aggro and makes them invisible. If you have attack on though, you will reappear right away. So, for my rogue there is a hotkey which turns off attack and then triggers the Escape AA.