It shows each person's present duty station and PRD, their prospective duty station (when there is one), and their last few as well. The big versions can have several codes for professional schools, qualifications, masters, etc., so it can also be an, uh, treasure trove of privacy sensitive information. It's also really valuable for JOs and DHs to find out about some of the more off the beaten path stuff. Detailers and community managers sometimes like to control that information for different reasons (some good but some selfish too).