You can make your own custom fields available for reading or writing on your payable invoices and payable credit notes, and expose them to the FinancialForce API.
You must create an integration rule (Type 1) to expose your custom fields on the payable invoice and payable credit note objects. See What is an Integration Rule? for more information.
Where are my custom fields?
In Classic Edition
The view of the application that you see when you first install the base managed package., click the 
icon next to the Account field to complete custom fields on the document header, and click the icon on the expense or product line to complete custom fields on the line. When you click theicon an expander panel Panels that are initially hidden when the page is loaded. They allow you to enter, and subsequently view, additional attributes of the related field or line item, such as further analysis or a line description. opens. To close the panel, click the
icon.
In Extended Edition The view of the application that you see when you install one of the extension packages (such as the Cash Entry Extension package). For invoices, credit notes, and journals Extended Edition is integrated into the main package, but must be configured., custom fields on the document header are available for input along with other header fields. To make custom fields on expense or product line items available for input, your administrator must add the custom fields to the appropriate Manage Lines field set.
When using custom input forms, your administrator must add any custom fields for both document header and lines to the appropriate input forms.