Sentry Drones are more specialized drones designed for sniping, taking up the same space as heavy drones and requiring the same bandwidth.
While the other drones orbit their target at high speed, sentries sit where they are deployed from and fire at enemies from long range. They have a long optimal and falloff as well as more damage and durability, but they have very low tracking speed and are immobile once deployed. Because of this, Sentry drones are best used against large targets or from long ranges.
A sentry drone cannot attack targets that are outside its owner's drone control range, even if they are within its own optimal range.