That's interesting. In Poland invoices also have to contain an auto-incrementing component (so you can't backdate something and slip it in, or post an invoice t

That's interesting. In Poland invoices also have to contain an auto-incrementing component (so you can't backdate something and slip it in, or post an invoice to a client but skip it when reporting taxes). However each company can design multiple "series" of invoices, for example per-client (e.g. 1/2019/MSFT, 1/2019/AAPL) or per type of services (1/2019/IT, 1/2019/Design) or office location (1/2019/CityA, 1/2019/CityB) or basically whatever. The year is optional, numbers could increment indefinitely, but it's convenient to reset the counter each year (or each month for large companies).