Unless something has changed, it was defined before
Discussion
The formal spec may have been removed amid edits and shuffling. Some event kinds have specific d tag usage defined, but there's no formal standard for all events, that I could find.
I don't know, I'm thinking it's a short uuid,
but in wiki format it's like the title https://github.com/nostr-protocol/nips/blob/master/54.md
The wiki spec uses a title-like format because it can be used to build human-readable URLs within the client.
Also never found one. I've read all of the NIPs, multiple times. Don't recall seeing one.
oof... im not seeing clear definitions either. The readme notes it as defined in nip01 but with the rewrite theres no info there. other nips reference in examples, but assume its been defined elsewhere.
it is an identifier with a single string value (not an array), so only the first item in the tag should be considered.
used as an identifier for replaceables. its also referenced by a tags like a surrogate primary key ({kind}:{pubkey}:{dTag})
Now my question is how relays handle it. If they index the whole d tag array, rather than just the first value, than an event which, for whatever reason, has multiple values in the d tag array would be impossible to access via parameterized lookup.