Engineer droids were B1 battle droids specially programmed to perform as battlefield technicians, pilots, and demolitions experts for the Separatist Droid Army. They were distinguished by the yellow markings on their head and shoulders, similar to that of the OOM command battle droids (sometimes they had the appearance of a B1 battle droid). Their programming allowed for the operation of virtually every vehicle-allied or otherwise in existence. They would usually engage their targets from close range.