MST is an acronym for Mercenary Service Track, which is the process by which human mercenary companies train and develop new recruits.
From the Books: "'I’m sure your teacher has explained why I am here. MST, which stands for Mercenary Service Track, is your pathway to fame, fortune, and a brighter future for all humanity.' Jim remembered smiling. It was the same catchphrase used in TV commercials promoting MST all the time. 'There are a lot of exciting classes in your future, and I’m here to help guide you through them. I’ll answer any questions you may have and provide you with whatever help you need, should you decide to continue with MST later in your schooling.' She looked around the room, seeming to make eye contact with each of them in turn."
- Cartwright's Cavaliers, p. 11.