What a W-9 is for
Form W-9 is used to request the taxpayer identification number of a U.S. person, including a U.S. resident alien, together with certifications and tax-classification information. Businesses commonly collect W-9s from U.S. independent contractors and vendors when payments may need to be reported on an information return.
Collect it during onboarding
A W-9 is easiest to obtain before the first payment, when the vendor has an incentive to complete onboarding. Store the form securely because it contains sensitive taxpayer identification information. Do not copy the TIN into casual notes, email subjects, or shared spreadsheets that do not need it.
Do not edit the vendor's tax classification yourself
The payee certifies its own information. If the business name on the invoice differs from the name on the W-9, use the W-9 and current IRS instructions to determine what information belongs on the information return rather than guessing from branding.
U.S. and foreign payees use different documentation
Form W-9 is for U.S. persons. A foreign payee generally provides an appropriate Form W-8 series form instead. Vendor onboarding should therefore ask the status question first rather than sending Form W-9 to every payee in the world.
Retention
The IRS independent-contractor guidance recommends keeping the contractor's W-9 in your files for four years for future reference. Tie the stored form to the vendor record so it can be retrieved during 1099 preparation without exposing the TIN more broadly than necessary.