Set the VAT profile: small business, cash basis and return period

Open Settings, the tab Company profile, section Tax profile. Owners, admins and accountants can change it; other roles see "Your role can view the master data but not change it." Choose Save at the bottom of the tab; Saved confirms. The VAT figures are calculated from the bookings when you open them, so a change applies at once.
Taxation basis: Sollversteuerung (Accrual basis (agreed consideration)) counts a sale in the period of its document date. Istversteuerung (Cash basis (received consideration)) counts a sale when the money arrives: each payment moves its share of the VAT into the period of the payment date, so a part payment is counted part by part, and a discount or write-off of a sale then counts nowhere. Input tax always follows the invoice date. The app does not check that the tax office allowed the cash basis; set what applies and confirm it with the tax advisor.
VAT return period: Monthly, Quarterly or Annual (no advance return). It decides which period the VAT return view opens on (the last finished one), which return the page Tax shows as next with its due date, the list under Deadlines and the period of the ZM line. For Annual the Tax page says there is no advance return. The period list in the VAT return view still offers months, quarters and years, so you can open any other period.
Small business (section 19 UStG) (Kleinunternehmer): new invoice drafts get the tax treatment Small business (§19), and finalizing refuses any other treatment. The reverse is refused as well: a client that is not marked as a small business cannot use the §19 treatment. The VAT return view then shows "usually no VAT return is due" and the OSS counter notes that OSS usually does not apply. The box does not switch off the VAT return calculation.
Compute the VAT return (UStVA) and ZM and export them as files (draft) is off for every client until you tick it. While it is off, the VAT return view says that the VAT return and ZM are switched off and offers Switch on in settings; the Tax page says the computed return is off and that the ZM is computed once it is on; the UStVA working paper and the file downloads are unavailable. Switched on, the figures are still a draft: the return box mapping is not signed by the tax advisor and nothing is sent to ELSTER.
These boxes exist only for clients with German tax data. A Swiss client sees VAT return period under the name "VAT (MWST) return period", has no small business box and no VAT return switch, and the VAT return, DATEV export and the other SKR03-based functions are not available for its chart of accounts.
The page Tax shows the chosen Taxation basis and Profit determination at the bottom as a quick check. See Look up your VAT return (UStVA) for reading the figures.