The mission difficulty relates to the dungeons where you can find them. For instance a Very Easy kill mission will have targets from Very Easy dungeons. So in the rat example; Very Easy Sewers.
I'm working on mission descriptions to say exactly which dungeons, but here is the sectors:
The difficulties relate to the city sectors (in Neocron City anyway):
Very Easy = Viarosso
Easy = Plaza
Normal = Pepper Park
Hard = Outzone
Very Hard = Wastelands