Thank you for your various questions and for sharing your situation with us.
Here is more information regarding the purpose of the Tax Identification Number (Steuer-Identifikationsnummer): https://www.bzst.de/EN/Steuern_National/Steueridentifikationsnummer/steuerid_node.html
As they mention on the website:
"All individuals receive an 11-digit identification number to ensure their unique identification within the tax system. The number will not change if you move home or marry. It will remain with you for the whole of your life. It does not contain any information about you or the tax office responsible for your tax affairs. Introduction of the number will allow for an improved and more efficient use of electronic data-processing."
"The identification number will replace the tax number in income tax matters. The identification number must be used on all applications, declarations and communications directed to the German revenue authorities concerning income tax.
To make things easier, for the time being, please additionally quote your existing tax number. For other taxes (e.g. VAT, motor vehicle tax, inheritance and gift tax, real property transfer tax) please continue to use only your existing tax numbers.
I hope this information helps. Feel free to share with us how your case turned out and what status you receive once you had the second appointment!