Document Reference Codes

CODE	Reference Number
PO	Purchase Order
IK	Invoice Number
SI	Shipper’s Identification Number
BM	Bill of Lading Number
MB	Master Bill of Lading
CN	Carrier Reference Number (PRO)
CO	Customer Order Number
CR	Customer Reference Number
CG	Consignee Order Number
6A	Consignee Reference Number
ACL	Application Transaction Reference Number

Source: ASC-X12 Data Element 128 - Reference Identification Qualifier.