Their original purpose was to identify messages in Apollo's distributed computing architecture. UID and later UUIDs were a reversible way to mark an intersectio

Their original purpose was to identify messages in Apollo's distributed computing architecture. UID and later UUIDs were a reversible way to mark an intersection point between two dimensions. Any two machines would generate the same UID/UUID for the same two inputs, and a recipient of an identified message could reverse the identifier back into the original components. They were designed as labels for ephemeral messages so the two dimensions were time and hardware ID (originally Apollo serial number, later Ethernet hwaddress etc). I think a lot of the confusion can be traced to the very earliest AEGIS implementation where the Apollo engineers started using “canned” (their term, i.e. static or well-known) UIDs to identify filesystems. Over time the popular usage of UUID fully shifted from ephemeral identifiers where duplicates were intentional toward canned identifiers where duplicates were unwanted and the two dimensions were random-and-also-random. The history gets even more complicated because Microsoft hired one of the top Apollo guys to do MSRPC for Windows NT, so there is also “GUID” which differs from UUID in the layout of the fields and is not mixed-endian despite what a lot of sources will tell you. In addition to ephemeral RPC message-identifying GUIDs Microsoft are also in love with canned GUIDs for identifying COM classes, media codecs, and almost anything else that would ever need a well-known identifier. See https://gix.github.io/media-types/ https://gix.github.io/media-types/ for example. Apologies for linking my own repo twice in the same comment section but I started (and need to get back to) compiling the history of all this in the README of my UUID library. Apollo started in 1980 and the Leach/Salz UUID RFC draft didn't happen until 1998 so there is a huge amount unsaid by the modern standards: https://github.com/okeeblow/DistorteD/blob/NEW%E2%80%85SENSATION/Globe%20Glitter/README.md#history https://github.com/okeeblow/DistorteD/blob/NEW%E2%80%85SENSA...