A "VAT" number is a "value added tax" ID number that is used for taxation purposes by countries that are in the European Union. Each country has its own, unique multi-digit code that must be used if you're filling out different types of tax papers in the country you live in. If you're not sure if the VAT number that you have on file from the documentation you received from your employer is correct, you can check easily using the Internet.
Open the Web browser on your computer.
Go to the European Union VAT number validation site.
Use the drop down menu to specify the country in which you live.
Type the VAT number you have on file into the appropriately labelled box.
Click "Verify." This will run the web utility that will check the validity of the VAT number you provided.