The Separatist Sub-Carrier functioned as a carrier vessel that operated underwater.
This Sub-Carrier had the capability to carry Mini-subs, which were piloted by single B1-Series battle droids, along with autonomous manta droid subfighter units.
The exit point for the mini-subs was through tubes situated on the lower section of the carrier. This launch process presented a hazardous situation, as Galactic Republic SCUBA troopers were able to stage ambushes against the mini-subs as they were exiting, and the subs had no option to reverse course once they began moving through the launch tube.
The general shape of the Sub-Carrier was similar to a vulture droid, but its overall dimensions and internal space were significantly larger. The sheer size of each carrier allowed for entire battles to take place on the ship's upper surface.
The Xi Char gave the Trade Federation the Sub-Carrier schematics at no cost, because they were unable to replicate the transformation ability of the vulture droid in their manta droid subfighter design.
The Separatist Sub-Carrier bears a resemblance to a destroyer droid carrier concept that was ultimately cut from Attack of the Clones, the key difference being that the unused concept had four legs instead of the two seen on the Sub-Carrier.