Core answer
Start here. This is the direct spoken answer to practice first.
Overview
Identifier choice affects more than uniqueness; it changes when an ID exists and how inserts can be sent.
IDENTITY lets the database generate a value during insert, while SEQUENCE obtains values from a database sequence and can allocate them in blocks. TABLE emulates a generator through a table and can become a contention point. UUID or application-assigned IDs exist before insertion, and AUTO lets the provider choose. The best strategy depends on database support, portability, write volume, and whether IDs are needed before flush.