what is the purpose of the account on an expense report header level when you have expense categories on the sublist?

To impact specific GL expense account

how does that impact differ from the accounts assigned to expense categories?

In subsidiary level you have to define the default payable account for the expense reports. Also each sublist expense category accounts are for debit and header level GL account should be for credit. In accounting you cannot just debit accounts without crediting another account and credit = debit is a must, Once expense is approved it moves to defaults payable account defined at the subsidiary level. I hope that make sense

nice thanks....i deduced that from the GL impact however what threw me off was the non reiumbursable expenses....it just credits and debits the expense category account while reiembursable does what you stated

Glad to help