Where the model requires the user to enter a specific input, it may be useful to format the cell such that it only accepts a specific range of choices. This offers two advantages: i) it prevents the user form entering a value that would not be accepted by the model and ii) it makes the model more user friendly as the user can choose among a predefined list of choices.
1. Select the cell where the user must enter an input value
2. Go to Data / Data Validation / Select "List" in "Allow value" / Specify the range of the possible values