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})