Pointers for Using P4780 / F4780?

Any pointers for using P4780?  We would like to use it for cross referencing our JDE ship-to numbers from the address book to the address code that our customers send on their inbound EDI purchase orders.  We could also use it for outbound invoice and ASN.

I'm puzzled because it seems to require two different AN8 entries, one in the header and one in the detail.  What is the difference?  What is intended in both of those entries?  They are both validated as address book numbers.

There are many interesting fields on the table that aren't accessible, but can be accessed through database read/write, of course.  Any help would be appreciated.