From OpenStreetMap Wiki
|for the VAT identification number|
|Used on these elements|
|Status: in use|
|Tools for this tag|
ref:vatin is a tag to add theon Wikipedia (usually the number of the operator found on the receipt you get when you pay something). The usual syntax is <country-code><VAT-number>. See the linked wikipedia page for more information and the codes.
You can attach this key to every business, usually it refers to the operator of the place. Some examples: