UUIDxxxxxxxx-xxxx-Mxxx-Nxxx-xxxxxxxxxxxx (36 chars, 32 hex + 4 hyphens)
M (version)4th group, 1st char = version number (1-7)
N (variant)3rd group, 1st char = 8,9,a,b → RFC 4122
ULID26 Crockford Base32 chars. First 10 = timestamp, last 16 = random. Lexicographically sortable.
NanoIDURL-safe chars [A-Za-z0-9_-]. Default 21 chars. ~2.7× smaller than UUID for same collision resistance.
NIL UUID00000000-0000-0000-0000-000000000000. Used as null/default value.