Use cases description is difficult but essential. It describes the process and functional application of tools. In our world, these applications are often IT functions. It is by no means the intention to mix this functional approach with the technical realization. That is the conversation between the system designs and the architect/developer.
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True greenfield development is like a air. You know it exists but you cannot see it. In system design there is always a legacy that needs to be taken into account. The greenfield approach however means that you always need to consider: what if we could do it again based on everything we know now