What is TAX Registration number format for Suppliers from Switzerland.

One of the suppliers from Switzerland is trying to register in oracle supplier portal but when giving all the basic details and click on next getting error:

ERROR: The tax registration number is already in use by another supplier for the same period. Enter a different tax registration number

There is no duplicate registration number & Supplier is registering first time in oracle.

Please suggest the correct format that can be accepted by system if the supplier is from Switzerland.