The Online checkbox on the DL Properties dialog defines whether reads are to be performed for the Data Location.
If the checkbox is not checked, then no reads will take place for the DL from any of the SDRs within the Sapphire Collector.
Tip: Turning a Data Location Offline is a good way of isolating the load on the underlying OPCServers. This can be useful when analyzing OPCServer performance and the impact that the Sapphire Collector is having upon it's loading.