W-9 tax form

Formally it is the Request for Taxpayer Identification Number and Certification, and it exists so a payer can report what it paid you and knows whether anything has to be withheld. US persons and US businesses complete it, including a single-member LLC or a sole trader using their own social security number. Creators outside the US complete a W-8BEN or W-8BEN-E instead. It goes to the sponsor rather than to the IRS, so treat it as a document carrying your tax ID and send it through something better than an email attachment. Signing one deducts nothing from the payment; it is reporting paperwork rather than a withholding instruction.