Creating new parameters question

[Adrianis]

^ this, but also...

how do I know that the new parameter will do exactly what I said

The parameter will do what you tell it to do in the logic of the function. Parameters are simply variables of a given type that you provide to the function when you call it, they don't 'do' anything in-and-of themselves

Is it tested that it'll do exactly what I describe?

You haven't 'described' anything for it to do. When you write some code in the function, you should have a good idea of how to test if it's working