Joining PO and Invoice data

Hi All,

I am looking to join PO and Invoice data using a third party program within a FICA set-up.  The purpose is to find if Invoices are created before PO's.  I generally do this by using the EKKO/EKPO tables and either the BKPF filtered by doc type (although FICA Invoice doc types are alluding me), or with the RBKP tables.  This doesn't seem to be as effective, therefore:

Can someone please help me with the following:

1. For FICA what tables contain PO header data

2. For FICA what tables contain Invoice Header data

3. How are these tables joined?

4. What are the invoice doc types in BKPF for FICA.

Thanks so much!