As the difficult descriptions say, you get extra loot at the end of missions based on how many exos you have standing at the end (usually one per exo, though I think the lowest difficulty level might have two?).
Near as I can tell, the type of mission influences what the loot is (computer parts, in this case) and rarity level (command centers give the best, heh).
The part you got did have an upgraded rarity level (green and white both being lower, iirc). The mark Foo level is mostly influenced by days passed... I think. And there's some randomization to it regardless.
Hope that helps, yeah.