Service Class User
SCU is an acronym for "Service Class User" which is the weird DICOM term for a client application. All DICOM network communications occur between an SCU and a Service Class Provider (SCP)
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.