These Training Droid units, much like the B1 and B2 versions, was used by the Galactic Republic on Kamino to train clone cadets.
Droid Design
BX-training droids were similar in appearance to the BX-series droid commandos used by the Confederacy of Independent Systems during the Clone Wars. The only difference between the standard commando droids and the training droids, was that the training droids were grey in appearance, and had orange target marks all over their body to show clones the best spots to shoot an battle droid.
Droid History
Although a few BX units were used on the ground during training simulations, most of the commando droids were placed inside small towers and operate a stun gun to try and take out clone cadets. Like all training droid units, the BX's weak spots were seen covered in orange targets and could be shut down using a button found in the kaminon command center.