This module calculates Washington State Sales Tax for the Ubercart module. Washington State has destination-based sales tax, meaning the seller has to charge sales tax based on the location the product is delivered to, rather than based on the location of the store/business. Thus, the tax rate is variable, and because there are literally hundreds of possible destination rates a simple sales tax setup in Ubercart will not work for Washington.
In this module, Tax is calculated based on the purchaser's shipping address (or billing address, for non-shippable purchases), and the tax rate is found via address lookup on the Washington State Department of Revenue's web site, so that it always uses the most recent tax rate information.