In HIL, as in any other scripting language, you can define as many variables as you like. However, certain variables names have special meaning.

Input Parameters are variables that you can change to affect the behavior of the solver.
Output Parameters are variables that the solver assigns values to in order to communicate information to you.
Solver Macros are snippets of code that you can execute (or that the solver might execute).
Command Variables are heisenberg variables that cause the solver to perform actions when you evaluate them.

A list of all solver parameters in all four categories is below: