There is a check box on the Properties dialog for contained module blocks for specifying whether or not to allow a variant module to be substituted on module instances. (By default the box is checked.) Sometimes individual units are not identical in every way to the unit class from which they are derived. For example, some units may have a slightly different type of outlet valve that nonetheless performs the same function. To accommodate these differences, you can use variant module substitution, which lets you substitute any module that has the same top-level interface parameters as defined by the module class. (The substitute module can have additional parameters, but must have at least all the parameters of the original module). The module substituted can be class-based or non-class-based. To do the substitution, simply delete the module instance in the unit module and replace it with a module of the same class as the original by editing the module block properties and defining the bound module path. When you delete the module block instance, the block name is retained and marked by <unbound>. Copy and paste to the area or process cell a copy of the module you wish to substitute. Open the Properties dialog for the unbound module block and, under Bound module name, browse to the name of the module you want to use.