The Identifier Landscape
One security carries many IDs because issuers, issues, venues, and vendors each mint their own keys.
Every security is described from at least four distinct perspectives, and each perspective mints its own identifiers. The issuer view asks which legal entity stands behind the instrument, the issue view asks which specific claim is meant, the venue view asks where it trades, and each data vendor decides how the instrument displays on screen. Security masters and reference-data services exist precisely to reconcile these views into one golden copy. Once you see an identifier as an answer to a specific question, joining datasets becomes a deliberate mapping exercise instead of guesswork, and the chronic pain of symbology mismatches stops looking mysterious.