As noted above, you can define limiting values or ranges for any variable. In the Equation Editor there are separate windows for these two values. In the Text Editor, the ranges are defined on the line containing the units of measure by enclosing the lower and upper limits in square brackets [LOWER, UPPER] after the units. During simulation, Vensim will report any violations of these limits.
sales = Salesmen * sales productivity
~ $ / year [0,1.0E6]
Dollar volume of sales
In this example sales is defined to be in the range of 0 to $1 million per year. With this specification, any points in time where sales are above $1 million (or negative) will be reported.
The motivation for using a range could be uncertainty of the equation's accuracy outside the range, implausibility of going outside the range, or interest in knowing when the range has been exceeded.