Consists of 1 or more stars, plus 1 or more planets. Non-UWP generated star systems could have zero planets.
The main world is the world described by the Traveller UWP. It is the world with the best star port.
A system may have several smaller colonies, which can be mining colonies, research bases or just places for people to live.
Only nice worlds will have standard colonies.
Colonies may be controlled by the main world, or be external and/or independent. A low tech world, or one with a poor space port, will not have any colonies of its own. Otherwise, it is assumed colonies in a system are children of the main world.