Event Class Category - Impact of Using 'Event Class' or 'Tax Event Class'

Hi,

I have setup the Determine Tax Applicability Rule as follows:

- Event Class Category: Tax Event Class

- Event Class: Purchase Transaction

The tax is calculated when entering a standard supplier invoice but NOT when entering a prepayment for the supplier.

I saw then that if I choose

- Event Class Category: Event Class

- Event Class: Prepayment Invoices

My questions are:

- What option shall I select if I want invoices calculated for AP standard invoices as well as AP prepayments?

Why when choosing Tax Event Class and Purchase Transaction the rule does calculate taxes for the AP standard invoice but not for the prepayment?