Starship missions are specified by a mission number (M), a "tow" number (T), and an "intercept" number (I). When the mission is Tow, the "tow" number specifies the ship to tow, likewise for intercept. Add-ons can provide additional missions. To set these, you must use a program that supports "M.I.T.", that is, it lets you enter the mission numbers and arguments manually, instead of restricting you to the handful of standard missions.