Cargo Bays

Large internal areas of the ship can be assigned to store cargo. The Federation uses standard cargo containers which are 1 metre cube. These can be stacked two high, but space must be left between them for moving, therefore it averages out that for each square metre of floor space, you can store 1 cargo container. 

Goods are noted in how many containers worth. For example, a trade might net you 123 containers worth of food.

A Galaxy class ship has four cargo bays. Each bay can store 200 cargo containers. Each Cargo bay has a hatch to the outside of the ship (for manual loading) and a semi-permeable force field (like the shuttle bays). They have almost exactly the same functionality as the shuttle bays and can be used in exactly the same manner. 

Dev Notes

The only real difference with regards to the simulator is that the cargo bays do not have tractor beams and cannot be used for shuttle landings due to the door height.