Suspension of registration halts issuance of tax invoices and filing of returns pending cancellation proceedings. Rule 21A provides that registration is suspended when cancellation is applied for or when the proper officer believes cancellation is warranted, including suspension triggered by return data anomalies or contraventions of registration conditions; suspended persons shall not issue tax invoices or charge tax, need not file returns, and may be barred from refunds during suspension, with suspension revoked on completion of cancellation proceedings or earlier revocation or upon specified compliance or filing of pending returns.