As association is the DICOM term for a network connection between two applications, one of which is the Service_Class_User (SCU - client) and the other is the Service_Class_Provider (SCP - server). It is basically a TCP connection, but with extensive negotiation between the two entities to agree which SOP_Classes and options they both support.
Since a DICOM association is a network connection any number of independent operations may be made over it. These operations are completely independent and potentially of utterly different types. With the exception of printing, no dependency or connection between the operations may be inferred.
Use in DicomObjects
Many of the "simple" operations in DicomObjects such as DicomImage.Send or DicomQuery.DoQuery (DicomQuery.Find in .NET_version) are designed to use only one operation per association, but it is possible to be more flexible (and sometimes more efficient) by using the methods based on the DicomConnection (COM_Version) or DicomAssociation(.NET) object, which allow multiple operations per association