Ideally, it includes your actual data including vendor master data, transaction data, product codes, reports and the like. In other words it simulates your actual operational needs. It does take time prepare a scripted demo and it takes time for the vendor to respond. It goes without saying that the vendor who puts the most into scripted demo has prequalified himself as someone you would like to do business with. Pay attention to the little features as well as the big ones.
Some systems are decidedly better at doing that than others.
Voiding invoices, vendor checks or purchase receipts may be quite easy or a royal pain depending on the product. These are repetitive type transactions that you want to make easy for operating personnel. The best way to uncover the less visible, but important features is to talk to operating personnel. They are well acquainted with the nuisance items and will love to tell you about them. Let's say you have a very complex commission structure which involves unusual calculations.