RML organizes requirements into objectives, people, systems and data. All of these areas are useful for articulating requirements in an RFP. We have a ton of information on RML (requirements modeling language) on our blog as well as some info on RFPs.

1) objectives - ultimately you want the system to achieve measurable objectivesl, making those objectives part of the RFP is very useful

2) people - including process flows - this will help you to drive the respondents to show you how the system algns with your process. If it is too different from your existing process you will have an uphill battle with your end users

3) systems - a diagram which shows all the interconnected systems and the interfaces that they will have to create to link in the new system

4) data- one of the biggest gaps is incompatibility of data models. Diagram your data using a business data diagram, your data elements using a data dictionary and then have respondents create a gap.

Requirements and business rules that need to be supported are derived the above models and provides a very complete picture to vendors.