Answer
At minimum, buyers should map order flow, inventory movement, purchasing, financial close, reporting dependencies, and any custom workflow leadership would consider unacceptable to disrupt. That documentation does not need to be perfect, but it does need to be honest enough to show where the ERP still fits and where the business is compensating manually.
Without that map, teams compare demos against assumptions instead of against reality. ERP decisions get better when the process truth is visible first.