TAXUSJ & TAXUSX are both jurisdictation based tax procedures.
TAXUSJ is used when you prefer to define your own jurisdictions in the system for tax calculation. Tax is calculated based on the Address & Zip code. For each Zip code you need to create jurisdiction codes in your system. TAZUSJ is internal tax procedure where you decide not to use external system
Whreas
TAXUSX is used when you hire a third party to calculate tax for you and return the tax amount to your document before the posting. Vertax, Sebrix, Sales Tax office are some of the external tax engines you can use. In this case the external party will help you in configuring the minimum set up they need to talk to you system. The main config here is Setting up RFC destination between both the systems so that they can connect to each other. I hope this helps