Classes marked with an (S) are service classes, which mean that while a member of that class, you serve full time in that group. These an only be taken during prior service (adventures would get prety dull if you were the only member of the group who owed full time service to the Army, and therefore couldn't adventure) and you must complete at least one term of your prior service before switching classes. OTOH, you must multiclass to another (non-service) class before beginning play.