The difference between Unit Level and Systems Level with respect to Cost Estimating can be found on page 4-2 of the Operating and Support Cost Estimating Guide.  The excerpt that follows is from aforementioned reference.  “In addition, where appropriate, the data should be provided separately for operational units and dedicated training units (e.g., Naval Aviation Fleet Readiness Squadrons). Where feasible, the data systems should provide users with system-level (i.e., end-item) data, as well as lower levels of data (major subsystems and components).”