The first 48 bytes of information in the data-container in question. If the action performed was a GET CONTAINER, it is the first 48 bytes retrieved. If the action performed was a PUT CONTAINER, it is the first 48 bytes of information to be placed in the data-container.
See CICS Service Flow Runtime data-containers for aggregate processing patterns for information on specific data-containers used by CICS® Service Flow Runtime at run time.