The Union One-Stop Shop (OSS) is a single EU VAT return designed to simplify the VAT return process for EU sellers. Extending the scope of the current MOSS scheme, OSS is split into two categories; Union OSS and Non-Union OSS.
You would be forgiven for assuming that the point of difference between the two categories is whether the seller is based in or out of the EU. To some extent it is, but the definitions are largely split between the types of transactions and businesses eligible to use each return.
Source Avalara
