Question:
Are there anybody standards for preparation this document?
Answer:
Check the ones at the SAP Solution Manager or the old ValueSAP. They come as part of the ASAP methodology.
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Answer:
the trick is to capture the requirements and to make sure that everyone agree to them. That's the most difficult part - to minimize any misunderstanding. Very often when two different people look at the same process written down in documentation, their understandings are completely different.
One of the standarts for requirements capture is UML, you might want to look at www.uml.org