When an account has been marked as a Tax account (see Section 18.3.3 on page 18.3.3, item Obsolete) several things happen:
The account will be shown in the customer and vendor account screens with a check mark to mark it “relevant for the customer”
The account will be shown in the part and service configuration screens with a check mark to mark it “relevant for the part/service item”
The account will be shown in the tax configuration screen in order to set a tax percentage on the account as discussed in the next section
By marking an account relevant for a customer, taxes will be calculated when creating a sales invoice for the given customer which includes parts which also have the specific account marked as relevant.