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UNID stands for “universal (or unique, some says) identifier”, which is required to define and store most in-game data (entities) except hard-coded elements (such as the opening screen, background graphics, etc.)
It is the most characteristic background mechanism of Transcendence, and you need to set and define these UNIDs when you want to add your own new elements (ships, items, star systems, anything!) to the game (as “modding”).
It is vitally important to avoid clashing UNIDs. UNID ranges are thus reserved for different purposes, and notably, individual users can reserve a UNID range for their own mods.
See the UNID_Database for details.