How's this: Let's take a complex process and dissect into it multiple subsystems and make it more complex to understand. If you are writing an ETL system from the ground up, this may be useful in determining what needs to be done. If you are using an ETL system that is already written (Informatica, SSIS, et al) then a lot of this has been done, and you don't really have to think about every little subsystem, just how are you going to use your ETL tool to complete the overall task of the ETL process to meet your business requirements.
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