A standard cargo unit (commonly abbreviated SCU) is a practical scale for representing the cargo capacity of spacecraft and starships, analagous to the cargo tonnage of ocean-going cargo ships. (FASA RPG module: Ship Construction Manual)

One SCU represents 6.75 cubic meters of cargo space. The mass of the cargo can vary widely, but modern impulse engines and warp drives tend to be powerful enough that volume is more of a limiting factor than mass: a ship can only carry what it can fit. In practice, in most cases a fully-loaded ship's mass will rarely exceed its theoretical maximum, rated at 50 metric tons per SCU (the mass of an equivalent volume of water being only 6.75 metric tons). (And in those rare cases, a safely-operated ship simply won't use all available space for cargo on those runs.)

The number "6.75 cubic meters" comes from the volume of a "box" 1.5 meters on a side and 3 meters (i.e., twice 1.5) tall, which are convenient dimensions for creating ship deckplans and other game purposes. A real-life 40-foot ISO standard shipping container has a volume of 67.5 cubic meters, or 10 SCU.
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