PS and Vertex integration

We have PS AP integrated with Vertex SaaS application so that we can have sales & use tax calculated properly. The key fields used for the calculation are the item ID and Ship To location, and GL account is secondary. We have our users who are very confused about using the Item ID - we have several hundred item codes. I'm interested in learning about how other PS customers, using the integration with Vertex, have configured their system...and if the use of the item ID is best practice and if others have similar issues.