A First Order Differential Equation can be used in a Function Expression or User Subroutines.

The DIF10 and DIF11 functions of are used to get the state variables in a Function Expression. For more information, see the System Element Functions for Function Expressions.

The SYSFNC function is used to get the state variables in a User Subroutine. For more information, see the Intrinsic Functions in a User Subroutine for User Subroutines.