During a simulation run, calculators in a model are run with given values for control variables to produce a set of outputs. Control variable values are saved as instances. A collection of a instances for all variables in the model is saved as a scenario. A scenario may be described as an instantiation snapshot. By using scenarios, a user can quickly recall a set of variable values used to create a given model output and can compare sets of values between different scenarios. There is a one-to-one relationship between an input set of variable values and the corresponding model output so that the output may always be derived from the input. The relationship among variables, instances, and scenarios is shown in the diagram below.

Key points to note:

• An instance is a value for a variable.
• All variables must have at least one instance.
• Variables can have multiple instances.
• For a simulation run, variables use one and only one instance.
• Saved collection of instances is a scenario.
• Instances do not have to be used in any scenario.
• Worksheet reflects the current set of instances being used.