The form a US business collects from each contractor or vendor before paying them. It carries their legal name, tax classification, and taxpayer ID. You don't send it to the IRS; you keep it on file so January's 1099s are accurate.
In Bizdivers: W-9s arrive by email or upload, AI extracts the data, and a missing W-9 can hold payments automatically. See Collect and manage vendor W-9s.