Sales-tax, VAT and indirect-tax engines that platforms embed to calculate, file and remit tax for their customers — Avalara, Vertex, Anrok, TaxJar, Stripe Tax, Sovos, Fonoa and the newer AI-native entrants. A vendor is counted only when a buyer discloses it. Tax vendors are not counted as buyers of their own competitors, and a company's own in-house tax product is excluded as self-operated.