The SCU (Service Class User) primarily sends DICOM requests to an SCP. It operates on a job basis doing the following:
Sends images out to external devices including printers
Copies images to other locations within a cluster
Retrieves images from remote destinations
Stores images to and retrieves images from a PACS Archive, if configured to do so
 | Note: The retrieved images arrive at the SCP. |
Before handling a SEND/ROUTE/COPY job, SCU checks if all the images for the job are in a cache somewhere within the cluster. If they are not, a RETRIEVE job is generated for the archive process and the job is not processed until the corresponding RETRIEVE job has completed successfully.