TTR Tax is an address-based tax lookup used for jurisdictions where a plain zip-code rate is not accurate enough (Colorado is the example Sprwt targets). It is layered on top of the standard state/zip-code tax tables: for the states you activate, Sprwt asks TTR for the exact rate for the delivery address, and falls back to the standard tables when TTR cannot resolve the address. You only need it if you sell into such a jurisdiction; otherwise leave the whole feature off and keep using the zip-code tax rates.