Cargo cans are 30-ton cylinders with (fairly) standard hookups and three rings of mount points (often with a hexagonal frame. Ships may be equipped with basic cargo-can racks, which include basic power/monitoring systems, or with more advanced racks that connect the can to ship life support, allowing stateroom modules. Door placement is also reasonably standardized, so that passenger modules can be connected to airlocks or ships' passages and cargo modules can, in many ships, be loaded/unloaded while in place.

Half-cans are 15-ton, half the length of standard cans, and have two mount rings. They can be mounted in full-can racks, but because they take two rings they often cannot be doubled up unless the ship is equipped specifically to do so.

Basic Cargo Can

Just what it says on the tin: a large, empty cylinder with temperature and pressure sensors. Many, but not all, have fire detection sensors, and some basic memory functions that carry ownership information and a record of what has been carried in the can - though that's more for the convenience of its owner than a trustworthy record. A shipper will be wise to assume the worst of a can they haven't had control of, and do their own radiation and contamination checks, not to mention pressure integrity.

Cargo cans generally have a loading door for package cargo, and frequently but not always a passenger-type door for inspections or hand-loading. They may have minimal heating and/or cooling systems to offset hostile conditions on-world.

Tanker Can

A variant on the basic cargo can, a tanker replaces the loading door with a pumping/valve system, generally specific to a particular type of liquid fuel. Tankers often have heating (or less often, cooling) systems to keep their cargos at appropriate temperatures when on-world.

Fuel Can

A specialized tanker with fuel scoops, to skim and refine water to refuel starships. It is generally mounted to a cutter, or to a multi-can scooper. Most starships don't hold cans in a configuration that allows scooping, but some smaller ships do.

Shuttle Can

A passenger module with basic passenger accomodations and baggage area, for in-system use. Some have sleeper compartments for multi-day trips. The module has a standard side-mounted passenger door, usually with airlock, for loading, and a front-mounted and rear-mounted passenger door to allow access to a cutter's cockpit and engine area.

Passenger Can

A passenger module for long-term accommodations: basically providing six staterooms plus about 6dt of baggage area. The can requires a heavy-duty power connection, but otherwise has self-contained life support. Like the Shuttle Can, it generally has three passenger access doors, and may or may not have a cargo door.

Can Traffic

Many basic cargo cans are all but publicly owned: passed from small carrier to small carrier at a deposit so small it's effectively a short-term rental. Many tramp freighters carry cans they don't own, and will readily abandon them in favor of a customer's can knowing they can pick one up at almost any port. Of course, cans with broken or non-standard connections are common, and some carriers deliberately alter their connections to make them less desirable for "accidental" re-use.

Highports impose fines for loose cans nearby, provided they can identify the culprit - sometimes tramp freighters discover they've been fined only upon return to a port, and after a sufficient period of time that they can't always contest the charge.

Many highports pay a bounty for loose can retrieval, and most systems with enough traffic have at least one crew with a rickety modular cutter collecting cans. Sometimes they're not picky about what constitutes a "loose" can.

Cans with more stringent requirements, such as those used to haul foodstuffs, will be tracked more closely, and are usually owned by a specific carrier or customer. This is also true of more specialized cans; one can sometimes rent a passenger module, for instance, but they require a much higher bond and are limited as to where they can be taken/dropped off, sometimes only back at the system where they were rented.