Unless there has been a recent change that I'm not aware of, there isn't a master BOL in E-Ship per se. One of our accounts has that requirement and our work around was to create a separate BOL for the trading partner that isn't location specific as the master. We used one of the custom fields on the BOL header for the master BOL number. The master BOL number is then entered on each location specific BOL that you create for the shipment to that trading partner. Where the master BOL number is needed in the ASN, the map pulls that number.
Where multiple sales orders with different customer PO's are shipped to the same location, one BOL per location can handle that and give you the details in the ASN file per PO number when you set that flag in the packing rule that I mentioned. Our account has both the master BOL and detailed ASN's and is handling both using the methods described here. This may work for you.
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