Provisional assessment: option where tax value or rate is uncertain; requires bond, security, procedure and interest/refund rules. Provisional assessment allows a taxpayer unable to determine supply value or applicable tax rate to seek provisional payment by applying to the proper officer, executing a bond and furnishing security not exceeding twenty five per cent of the bond amount. The rules prescribe a sequence of notices, specified forms for application, provisional order, calls for information and final assessment, and time-limited extensions that may keep assessment provisional for up to five years. Interest is payable on any shortfall from the due date; overpayments may be claimed by electronic refund subject to unjust enrichment rules and interest on delayed refunds. Security is releasable by application and officer order within a short prescribed period after settlement.