Are your users asking why the 9.1 BI_HDR page no longer has the 'Account' field?

Answer:

n 9.1 BI_HDR pages in Maintain Billing>Standard Bills> add new Bill.

have been  rearranged and refactored  to come to a cleaner 3 column look and feel.

This was achieved through eliminating redundant information from the Header page.
The account value and the other chartfields are available in the Header - AR Distribution page, which can be navigated from the drop down menu list,

so decision was taken to eliminate the account field and the AR Distribution hyperlink from the Header page.

This was done with assumption that with minimal initial training, users can access the Header - AR Distribution page to get to the same information.