Engineering Bags are a special type of bag that hold items created by engineers. They cannot hold many items used by engineers however, like ore, bars, gems, and cloth. Besides all your engineering trinkets and parts the bags also hold engineering schematics, and as of 2.3, all primals and motes.
The is a 24-slot Engineering Bag, that can be created by engineers in the Burning Crusade. The schematic is sold by Feera in The Exodar, Wind Trader Lathrai in Shattrath City, and Yatheon in Silvermoon City.