If you completye all objectives, you get the rot A cache reward. This is the same for all empyream missions.
If you do the (optional) side objective - the white marker in the derelict - then you get the rot B reward as a bonus. All you have to do is get to the marked terminal and hack it. There's no complexity to it.
Always do the optional objective first, as it is optional you cannot do it after completing all the other objectives. Personally I always do it as the first thing, its very quick to hop in there and then worry about the rest of the mission.