As far as I can tell, CSCI was defined in the same logic as HWCI (Hardware Configuration Item) in DOD-STD-2167A which simply defined CSCI as a configuration item.
More clear definition is in MIL-STD-498 which superseded DOD-STD-2167A:
CSCI – An aggregation of software that satisfies an end use function and is designated for separate configuration management by the acquirer. CSCIs are selected based on tradeoffs among software function, size, host or target computers, developer, support concept, plans for reuse, criticality, interface considerations, need to be separately documented and controlled, and other factors.
I think it would make sense when you map CSCI to a deliverable application. A CSCI will require basically one set of SRS and SDD. A System may comprise one or more CSCIs so other deliverable documents such as ICD and Test documents may exist for the system.