Field help:
(2018)
Your tax number (Steuernummer)
The current tax number can be found in the documents of the tax office, for example, on your last tax assessment notice (Steuerbescheid).

The tax number is issued by the tax office to every taxable private or legal person. The number is uniquely assigned to a taxable person.
This tax number should not be confused with your tax identification number, which is valid for your entire life. There is currently no information available about when the old tax number will be replaced.