UUID v4: Randomly generated, most common UUID version. 36 chars (with dashes).
UUID v1: Time-based, includes MAC address and timestamp. Sortable by time.
Nil UUID: All-zero UUID: 00000000-0000-0000-0000-000000000000.
ULID: 26-char time-sortable identifier. Crockford Base32 encoded, URL-safe.
UUID v4 — Database primary keys, API request IDs, distributed systems.
UUID v1 — Event tracking, log correlation, time-ordered identifiers.
Nil UUID — Default/empty values, placeholder identifiers.
ULID — Sortable IDs, event sourcing, CQRS systems, S3 object keys.