Solver Variables
Use the Variables tool to create solver variables that can be used to create an algebraic expression of state variables, as well as other solver variables. This can then be referenced in function expressions throughout the solver input file.
Create Solver Variables
Edit Solver Variables
Use UserDefined Properties for a Solver Variable
If desired, define the solver variable using the UserDefined tab, which will allow you to specify the properties of the variable using user subroutines.
 From the Properties tab, click the Userdefined properties check box.
 Click the newly added UserDefined tab.

Define the user subroutine.