FLU stands for front light, BLU stands for back light.
just got my BLU today and it's fantastic! Not sure how long the battery life is on it but its so clear!
P.S. had a FLU for a few months and that was great also but it developed a fault (front light stopped working ) so I sent it back to GBAX and they replaced it with a BLU.
Anyone considering buying one I would reccomend GBAX their after sales service is excellent!