If the NVOCC is an ACE participant, should we file the master bill of lading instead of the house bill of lading with the ISF?

No. The ISF must be submitted at the lowest bill of lading level (i.e., house bill or regular bill) that is transmitted into ACE. If the carrier creates and transmits a regular bill of lading number on behalf of a non-automated NVOCC or freight forwarder, the ISF filer must submit the regular bill of lading number that was transmitted into ACE as part of the ISF. A master bill of lading will always have underlying house bills.

From the Importer Security Filing (ISF-10, ISF-5) CATAIR Version: July 2012 Implementation Guide: https://www.cbp.gov/document/technical-documentation/ace-camir-importer-security-filing-isf-10-isf-5

Valid qualifier codes are:

OB (ocean bill of lading – used for regular bills),

BM (house bill of lading)

For purposes of the ISF, the following bill of lading definitions apply:

Regular bill – Defined as a non-master bill of lading with no underlying house bills. For ISF, this is the bill reported to CBP in ACE, which is also referred to as straight bill or simple bill. If multiple regular bills exist, the SF15 record is repeated accordingly.

House bill – Defined as a bill of lading issued under a master bill. For ISF, this is the house bill reported to CBP in ACE. If multiple house bills exist, the SF15 record is repeated accordingly.

Master bill – Defined as a bill of lading covering at least one or more underlying house bills of lading. If the filer chooses to provide the optional master bill for house bills provided in the SF15 record, the master bill is reported in the SF20 record with an MB qualifier. As with the SF15 record, multiple SF20 records are used to report multiple master bills.

CBP requires only the number for the bill of lading at the lowest level (i.e., the regular straight/simple bill of lading or house bill of lading) and not the master bill of lading number to be reported in an ISF.