On the State and Local Tax Deduction Worksheet there is a line 17, where state mandatory taxes is filled in - where does that come from?

The amount on the State and Local Tax Deduction Worksheet comes directly from your W2s, or estimated tax payments if you entered any.  

Taxpayers who itemize deductions on their federal income tax returns can deduct state and local real estate and personal property taxes, as well as either income taxes or general sales taxes. The Tax Cuts and Jobs Act limits the total state and local tax deduction to $10,000.

I found it was for other state taxes such as unemployment.

For me it was for a state transit tax listed on my W-2