Numbering sequences: identifiers people can read — It extends AutoNumber, it doesn't replace it

A sequence writes into an existing AutoNumber field. A field with no sequence attached keeps handing out plain integers exactly as before, so switching this on is additive. AutoNumber was chosen deliberately over a Text field: it is already server-owned, so no write path anywhere — grid, import, automation or public API — will accept a value for it. Nothing new had to be protected to make an identifier trustworthy.