Payable and CE business unit assign

Dear ALL

When we enter to the Receivables Billing we can see on the top of the Page they ask us to choose our business unit

but in the payables (Invoices) or Cash Management ( Reconciliation) we don't have this option

Also for the Payables creating an invoice we have as drop down all the business units how can we limit them to specific access for specific users ?

Thank you

Regards

Mickel