DCOM introduced the concept of Application IDs (AppIDs), which group configuration options for one or more DCOM objects into a centralized location in the registry. You specify an AppID by indicating its value in the AppID named value under the object's CLSID.
By default, an ATL-generated service uses its CLSID as the GUID for its AppID. Under HKEY_CLASSES_ROOT\AppID, you can specify DCOM-specific entries. Initially, two entries exist:
LocalService, with a value equal to the name of the service. If this value exists, it is used instead of the LocalServer32 key under the CLSID.ServiceParameters, with a value equal to –Service. This value specifies parameters that will be passed to the service when it is started. Note that these parameters are passed to the service's ServiceMain function, not WinMain.
Any DCOM service also needs to create another key under HKEY_CLASSES_ROOT\AppID. This key is equal to the name of the EXE and acts as a cross-reference, as it contains an AppID value pointing back to the AppID entries.