All the products share essentially the same interface and each is a superset of the one that precedes it. Vensim PLE Plus adds to PLE easier use of data, and Monte Carlo or sensitivity simulation capabilities. Professional allows you to use subscripts to represent more detail with less effort, and also supports optimization. Vensim DSS include the ability to construct management flight simulators and also to use external functions and compiled simulations. There is a comparison chart here.
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