The cargo module was an independent cargo container which was attached to a spacecraft at hardpoints. If the load in a module was not balanced, a vibration would result. One possible solution to the vibraton was to trim the load.
Cargo modules were a familiar sight on 22nd century Earth freighters. These freighters could carry eight cargo modules, like the Y-class ECS Fortunate, or carry ten cargo modules, like the J-class ECS Horizon. (ENT: "Fortunate Son", "Horizon")