Invoice a customer in the EU or abroad

Owners and admins finalize; accountants and bookkeepers can prepare the draft. The app invoices with German VAT or without VAT only: it has no foreign VAT rates (a rate not in force in Germany is refused), it does not file the OSS return, and it stores neither a proof of delivery nor export evidence. The BZSt check below is off by default.
Create the customer first, under Settings, Contacts or with Create new customer in the invoice editor: country, VAT ID and, in the editor, the Customer type Company or Private person, which decides e-invoice and VAT. The editor then proposes the Tax treatment that you may change: goods if at least one line has the type Material, otherwise services.
The proposal: a customer in Germany Domestic, VAT charged. A company in another EU state with a VAT ID of that state: goods Intra-EU supply, services Service to an EU business (reverse charge abroad). A private person in the EU, or an EU company without such a VAT ID: German VAT. Outside the EU: goods Export delivery, services to a company Service outside the EU (not taxable in Germany), to a private person German VAT. A small-business client always gets Small business (§19).
With Intra-EU supply and Service to an EU business (reverse charge abroad) the lines carry no German VAT, the PDF prints the customer's VAT ID and a legal note such as "Reverse charge: the recipient is liable for the VAT." Finalize is refused when the customer's VAT ID is missing, or "not valid for an EU invoice without German VAT" (a German or malformed ID): correct it on the customer or invoice with German VAT.
BZSt check of the VAT ID: switch on Confirm buyers' foreign VAT IDs with BZSt (section 18e UStG) in Settings, Company profile tab; it sends your VAT ID and the buyer's name and address to the Federal Central Tax Office and needs your own German VAT ID. On a customer under Contacts the block VAT on invoices to this contact has Confirm with BZSt and shows Valid, Not valid, BZSt not reachable or Not checked. Finalize asks BZSt itself and reuses an answer for one day. "Not valid" blocks; unreachable, not asked, or details that differ from the register only warn.
A Gelangensbestätigung (proof of delivery for an intra-EU supply) cannot be recorded in the app, and the invoice screen shows no reminder; only the agent mentions the missing proof on an invoice it drafts from a contract. Keep the proof with your records. The same holds for export evidence ("keep the export evidence"). If the agent drafts an invoice without a customer, German VAT is charged until you confirm the customer.
In a foreign Currency the VAT is converted to EUR at the ECB rate of the service date when you finalize, and the PDF prints it; without a rate finalize is refused. The OSS counter under Tax, Tax status adds up the net of sales to private customers in other EU countries per calendar year against 10,000 EUR, with a warning and the countries. It only counts (reviewed or posted bookings; not for small businesses); the OSS return is filed by hand in BZStOnline, and your tax advisor decides.