A self-assessed tax is one that you must pay, even though it is not collected by the vendor. These taxes commonly arise when you order goods from another state or province, but are still subject to local sales taxes.
Accounts Payable does not have a self-assessing tax type, but you can use the program to calculate self-assessing taxes and distribute them to expense and tax payable accounts. To do this, create two tax authorities: one for the tax expense, and one for the tax payable.