Tax Calculation

Hi

Just want to know how tax rate is retrieved in AP for an invoice.

We know its from EB Tax but we have two invoices with same Tax regime with different Tax rate.

We need to know on what basis the tax rate is determined n AP.

we use Vertex in AR for getting the tax, is that AP tax also come from Vertex.

Please help me getting a document to understand thsi flow perfectly.

Thanks

Nambi S