Local variables, by default, are created automatically for each parameter of a feature/solid and can be used when creating/editing equations for parameters, or when adding/editing constraints. You can toggle the display of local variables with the Locals setting in the equation dialog, which is opened when you click the equation icon for a parameter.
Local variables are available only for the selected solid. The naming convention used for local variables is:
_<Feature Name><Node Number>_<Parameter Name>
For example, a slab on node 1 of the feature tree would have the following local variables defined for each of its dimensions:
_SLAB1_Length, _SLAB1_Width, and _SLAB1_Height
Local variables for a solid always are available for use when creating or editing features on the selected solid. They also may be used in equations.