Missions are carried out on self-contained maps, after which you're back to the debriefing screen, where you allocate skill points earned, buy / sell equipment, then read the news and pick which next mission to go, repeat. Each mission map is developed by several sub-quests, which are available to you as you progress along that map. Often a base or NPC will initiate dialog with you, giving you a (somewhat limited) choice whether to help or not, which can reveal additional sub-quests. The maps are nonlinear in the sense that you don't get all the sub-quests in them from the start. As you fly around talking to NPCs or bases they will be revealed to you. Of course, most of the time you already come in with a goal in mind and the sub-quests are all already predefined - there are no random ones.