The Flax battle drone is a bot that has only one available slot. In that slot can be equipped any kind of laser cannon, shield generator, or extra to boost your ship in damage or hp. It is the only available drone that costs Credits. The Flax is usually for beginners due to it being easily attainable and only having one slot.
Once you have 7 flax drones, you should focus on bidding for an Iris drone in Auction. Once you get one, sell a Flax and repeat till you have 8 iris drones.
After an update, you can no longer profit from leveling up flax drones to their max level in order to get lots of credits.
The last part is not true if you had 5 flax drones.