Some items require specific containers. These items have container load relationships, which are direct relationships between a container item and a contained item. The relationship specifies how many of the contained item can fit into one container. You set up container load relationships in the Container Item Relationship window.

Although you can establish multiple relationships for a single contained item, the cartonization process uses only the preferred relationship. There can be only one preferred relationship for each content item.

Prior to setting up these relationships, the container items must be properly setup. Any container item setup using the steps above will is a valid container item for a direct relationship. Alternatively, to setup items just for direct container load relationships, the only required step is to check the container flag in the container object group. No other
physical attributes of the contained item or container item need to be entered.

more details @
http://www.oracleug.com/user-guide/order-management/managing-packingcontainerslpns