Each state sets its own forms and rules, though many accept multistate forms such as the Streamlined Sales Tax certificate or the Multistate Tax Commission's uniform certificate. The seller collects the certificate before or at the time of sale, checks it is complete and valid for that state and type of exemption, records which customers and states it covers, and tracks expiry dates, since some certificates must be renewed. In an audit, every exempt sale must be backed by a valid certificate, and missing or expired ones are one of the most common sources of assessments. For businesses with many B2B or reseller customers, managing certificates by hand quickly becomes unworkable. Exemption certificate management software collects certificates through customer portals, reads and validates them, links them to the tax engine and flags expiries. Avalara, Anrok, Zamp and Kintsugi manage exemption certificates within their sales tax platforms.