Applicant Checking... What is it used for? It's used to see who's online in your guild's "Requests" list.
It sits around, waiting for you to open your guild roster. When you do, it sends invisible addon messages to all the players on the list, and waiting to see if the system responds back that they aren't playing. After a set timeout period, the addon will update the status of all of the users on the list.
This addon is a must-have for officers and GMs of pretty much any guild. It does not drastically change the interface, and requires very little memory and bandwidth to do what it does (It sends "GAOC!" as the header and a 0x01 byte as the message.), so you don't need to worry about whether to leave it on or not.
There is a built-in throttle as well, to keep the bandwidth usage down. Each check is fired based off of a UI event, and the addon will only run a check if one hasn't been performed in the last 30 seconds
The addon has some pretty basic commands, and at the moment does not provide an in game help for them, so here they are /gaoc on
Turns on the applicant checking, and shows the icons in the default guild UI /gaoc off
Turns off the applicant checking, and modifications to the guild UI /gaoc force
Shows the recruits list, and updates the icons. /gaoc spam
Toggles the spamming of recruit-list updates.