It's all about timing. You can't go somewhere unless someone is leaving (usually), this isn't as big as a problem for say, the URL communities. You'll only get the option of what's available. Some communities, such as supply, give you PCS orders to school, so you're performance will effect your selection. However, for something like CEC where the school isn't a PCS, you'll fill out a "wish list" of 3 choices during OCS. You're going to have to decide what's more important to you, job or location, then list it. They'll try to match your preferences with their openings.