This is the seller's Social Security Number or business tax ID, used for year-end 1099 reporting. You enter the full number once; after that the screen only ever shows the last four digits (for example ***-**-1234). Pair it with the matching Tax ID Type so the system knows whether it's an SSN, EIN, or ITIN.
This is sensitive personal data. Only enter it from the seller's own paperwork, never guess, and don't share the full number. Once saved, the full value can never be read back on screen — you can only re-enter a new one.
Technical section
Data model, API, and internals — visible to developers.