Core concepts

• Architecture — how a query flows to an assessment, and the own-the-logic / source-the-data boundary.
• Regimes — the tax regimes shipped and how they are selected.
• Exemptions — express a buyer certificate natively and get a native Exempt assessment, applied deny-by-default over the regime's verdict.
• Rate breakdown — splitting an assessment's tax across the state/county/city authorities that levy it, with the parts summing exactly to the whole.
• Marketplace facilitator — when the marketplace is liable, the seller charges nothing and still reports the sale.
• Return data — aggregating assessments into per-jurisdiction and per-authority totals for a filing period.
• Dates — the tax point vs the reporting date, and why one date drives every dated lookup in an assessment.
• EU special territories — ten places inside a Member State where its VAT rules do not simply apply, and why a country code is not enough.