In this transaction you are able to assign country to a tax calculation procedure. There are countries as US for which few tax procedure exists and you have to consider which one suits best your needs. Also there are cases in which you could have to use a custom tax procedure which you have to assign here.
IMG Path: Financial Accounting (New) -> Financial Accounting Global Settings (New) -> Tax on Sales/Purchases -> Basic Settings -> Assign Country to Calculation Procedure
Table: T005, T005T
When you open the transaction, you have to find the country for which you want to assign or change tax procedure.