The forwarder or NVOCC issues the house BL to the exporter it serves, the ocean carrier issues the master BL to the forwarder that booked the space, and one master can cover the consolidated house bills of several exporters. On the desk that means two documents per shipment to keep aligned, with the actual exporter named as shipper on the house bill while the forwarder itself stands as the carrier's customer on the master. The split also decides who files what: the carrier files manifest data at master level, the forwarder's NVOCC desk files its own house-level data, and a gap on either side holds the cargo.
Glossary
House BL vs Master BL
The pair of bills of lading covering the same cargo: the house BL, issued by the freight forwarder or NVOCC to the actual shipper, and the master BL, issued by the ocean carrier to that forwarder or NVOCC.