When forwarding Sales order (COA) item(s) to an existing Shipping advice (CSA), some fields must match:
The Account, currency (CUR), Location country code: LOCCC, Payterm, Payby, Delivery terms (Delterm) and the Shipping address: SADDRID.
If a Credit card Transaction id (CCTID) is defined on the Sales order (COA), price agreement (PRICEAGR) or index agreement (INDEXAGR), this must match on the Shipping advice (CSA).
The Shipping advice (CSA) must have status 1.
If company settings are not activated that allow mixed locations, the location (LOC) must match.
It is possible to see this information by clicking on the info icon (I) in the forward pop-up too: