The 63rd floor is where challenges will take place. No missions will send you here thankfully. On the 63rd floor, you will generally be tasked with finding and defeating Heartless. Due to security purposes, characters will only be able to open 3 - 5 doors per visit. What this means is you'll have to think and choose carefully in order to find the Heartless and defeat them. If you fail, you could always retry the challenge at a later time.