if you keep gaining xp but don't actually level (as long as you stay below 888 in the fighting skills), you can keep taking the kill 7 droneII missions, which racks up a fair amount of xp very quickly in some sectors.

It cuts off when you reach enough exp for level 4 combat, and are then forced to do advanced missions which don't stack up.
Unless a dev broke it.

Uh, not drone1s, drone2s, this is the advanced missions, If I hadn't levelled up to 8887, I would still have been able to take drone2 missions (kill 7 drone2s), however I did, and subsequently I can't.