In the context of an invoice, TF stands for "Taxes and Fees." It is a line item on the invoice that lists all of the taxes and fees that are associated with the sale. These taxes and fees can vary depending on the location of the seller and the buyer, as well as the type of goods or services being sold.

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Note added at 3 ore (2023-05-20 19:01:02 GMT)
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The next line on the invoice should include the tax amount. Separating the taxed amount from the billed amount will be useful come tax season for both the invoice issuer and the invoice recipient. Any tax exemptions or adjustments should also be included. Be sure to include the applicable tax rate, whatever that might be. Most sales taxes are imposed on a state or local level, but other taxes and fees might also apply (and should be included in the invoice).

To summarize, if a transaction generates any sort of immediate tax obligation (anything other than year-end income tax), then the issuer should include all corresponding tax information.

https://fundbox.com/blog/tax-invoice/