All DICOM network communications occur between an SCP and an SCU

• SCU is an acronym for “Service Class User” which is the weird DICOM term for a client application.
• SCP is an acronym for “Service Class Provider” which is the weird DICOM term for a server application.

In the vast majority of cases, the Association is initiated by the SCU with the SCP responding, but DICOM does allow Reverse Role Negotiation whereby the initiator can request to be regarded as the SCP. In practice, this is only used as part of the Storage Commitment service.