You would have 700 Slaves as each 1 is in 1 Cargo container, just as any other commodity traded.Says who? If I buy 700 tonnes of slaves are you suggesting I have 7000 people on my ship? (700t -> 700,000kg -> 100kg per person -> 7000)
I say each person occupies 1 cubic meter, which could be thought of as a volumetric tonne.
If we use that notion then it makes a lot more sense that a ship that can carry 700 tonnes of vegetables or 700 tonnes of Gold in the same cargo space. If we go off weight the veggies would occupy a considerably great volume than 700 tonnes of Gold. So using the shipping model anything that has a volume of less than 1 cubic meter per tonne, i.e. Gold is measured by mass, anything that is greater is measured by volumetric weight, which equates to the volume occupied by a reference mass, often water, which is about 1tonne per cubic meter.
Note MASS not weight to account for differing gravities. Divide weight by 10-ish on Earth
Each container adds 1 tonne of mass to your ship
Jettison 1 unit of cargo, one cargo container is jettisoned, and the mass of the ship is reduced by one tonne
It is inferred that the container contains the commodity as well as any support needed to sustain the commodity in a vacuum without harm plus ballast
A universal mass and shape of a cargo container allows for easy of transport, un/loadings and load balancing