To ensure smooth processing upon arrival, there are specific system rules that must be followed when transmitting multiple Cargo Control Numbers (CCNs) using the IID. The ECCRD for the IID provides some guidance on these rules on page 25. Here are some of the key rules:

The IID allows a single EDI release transaction to contain multiple CCNs.
A minimum of one CCN must be referenced on the IID.
If the IID is submitted with a single CCN, the single CCN must be referenced at the Declaration level of the IID.
If the IID is submitted with multiple CCNs, the CCNs must be reported at the Invoice level of the IID.
All CCNs referenced on an IID must be non-consolidated documents.
All CCNs referenced on an IID submitted pre-arrival must arrive at the First Point of Arrival in the same conveyance.
If one CCN on the IID is referred for further processing or examination by the CBSA, the entire release and all CCNs associated with the IID will be referred.
All CCNs referenced on the IID that have been transmitted must be in “”arrived”” status before the IID will be eligible for release.
The maximum allowable CCNs referenced on the IID is 999.
For each CCN referenced on the IID, a corresponding cargo document must be transmitted, unless otherwise exempt.
Only one shipper and consignee can be transmitted per cargo document. For each unique consignee or shipper, a separate cargo document is required.