RAD is a hack by Pokeytax that allows you to change or add to the Job Wheel. Currently, any extra jobs added must share a JP pool and known ability bytes with a generic job. This means any ability learned in one job will automatically set the ability in that slot as learned in the other job.
It is notoriously buggy, so be sure to backup and use with caution
For more info, read this thread: http://ffhacktics.com/smf/index.php?topic=7031.0