This relation seems to exist in AX2012 R3 CU13 as well. But not in CU11 so it's a fairly new addition.
So the answer to the original question: the relation between CustPackingSlipTrans and SalesParmLine is defined in relation called SalesParmLine in table CustPackingSlipTrans. But only in AX2012 R3 CU13 and later versions.
But this relation points to the SalesParmLine which was used to create the packing slip. When you are creating an invoice this relation has no use for you.
But as Sukrut suggested, in the standard application you can select a packing slip that you want to invoice. So you should find a way to copy your fields from the packing slip to the invoice, by investigating the standard code.