As stated in the Your Role(s) in SF announcement in the MOS Selection subforum, soldiers get to choose two MOS's upon completion of In-Processing. Depending on the circumstances, you'll either get one of your selections, both of your selections, i.e. one being your primary duty and the other being your secondary duty, or you'll get selected for another MOS. Once you've graduated the Qualification Course, and have been a Green Beret for 2 months, you'll have the option to either change one or both of your selections, or add a third MOS. If you wanted to add a fourth MOS, however, you'll need to have served as a Green Beret for another 2 months before either replacing MOS's or adding a fourth one. Soldiers will also earn a Frontier Skies medal for their ASU if they've been trained in at least 4 MOS's.