The Assumptions input panel provides a number of important fields.
Rates of Return
This panel asks you to enter the nominal average rate of return for regular assets and retirement assets. Since you are building a financial model that is perhaps 20 years, 30 years, or more, you should enter rates of return that reflect your assumption about the returns on these pools of money over the life of your model.
You can also set Future Nominal Rates of Return, which allows you to set a year of change and a new rate of return at some year in the future.