DICOM services cover two worlds, the composite world (C) with its associated objects and services, such as images, and the normalized world (N) that contains services and objects used to manage those composite objects.  

Each SOP class is part of either the composite family or the normalized family. The names of the service commands provide a clue about which family each command belongs to because all composite services start with the letter C, e.g. C_STORE. 

Composite objects are also sometimes called "persistent" objects and are typically kept around, and can be queried and retrieved. It is not possible to delete a composite object, at least not using DICOM commands.