Default some custom fields from PO to Vendor invoice

Hi,

I need to understand the PurchFormLetter class hierarchy. I need to create a vendor invoice and pass few custom fields from PurchTable and PurchTable records in VendInvoiceInfoTable and VendInvoiceInfoLine.

While debugging the code I found that there is a child class PurchFormLetter_Invoice which is used to create invoice.

Can anyone let me know the working of PurchFormLetter for Vendor invoice along with the method where I can place my code.

Thanks.