The difference between a Primary Cargo Control Number (Primary CCN) and a Previous Cargo Control Number (Previous CCN) is as follows: 

Primary CCN: This is the CCN of the Cargo Control Document used to report the goods at the First Port of Arrival. The freight forwarder who has contracted with the primary carrier (e.g. the master loader) submits the required house bill data and then links it to the primary CCN submitted by the carrier. In a single-tier consolidation scenario, the previous CCN is the same as the primary CCN.

Previous CCN: This is the CCN of the Cargo Control Document that immediately precedes the document in question in the chain of cargo control. In a multi-tiered consolidation scenario, the previous CCN is the consolidated house bill transmitted by the freight forwarder above you. It is important for lower level freight forwarders or co-loaders to quote the Previous CCN on their House Bill Close Message and not the Primary CCN.