Thanks, @David Skowronek.  Our situation is that we are still early in our journey to migrate over to CDSM and at present have close on 10,000 Service CIs.  In time they will be reclassified to other classes but for now they are just plain Service CIs.  For maybe 10 years we've had a custom 'flattened' table of Service to CI relationships, so that predates Service Mapping and the svc_ci_assoc table.  That custom table is used for adding impacted services in Change and Incident, and also in other places across our instance, which is pretty customised.  So yes as you say, as we move over to using things like Calculated Application Services, we can hopefully use svc_ci_assoc instead of our custom table.  Until then, we have to keep using our custom table, and try to keep optimising the code as our CMDB grows in terms of number of CIs and relationships.  That's why I was hoping to get some useful pointers from the OOTB code that maintains svc_ci_assoc, which I could make use of in our own custom scheduled job.  However I can fully understand if it's not information that ServiceNow are prepared to disclose.  But it was worth a try!