The idea is simple; you can go to two separate places from here, either the Rank Up or Mission Payout areas depending on what transaction you need done, specifically.
If you're looking to rise in power and spend some EXP you'd head to the
[Rank Up] forum where you'll post what rank you currently are, what rank you're buying your way to and the total cost for that. A mod will come along and approve it, removing the experience from your account and sending you on your way.
Ability and item buying does not go here and stays within their respective areas. The transformations to Arrancar and Visored also go here.
If you're looking to be paid for a job well done and earn some extra EXP you'd head to the
[Mission Payout] forum where you'll post the link to the topic in which you carried out the Mission and a moderator will come along and approve or deny it. As long as you follow the guidelines you should be fine and the experience will be added to your account.
Happy hunting!