Very often, especially new Mathematica users stumble over the following error: They gave, maybe hours ago, a symbol a value like
x=3, and later they try to use it where a function really expects a symbol:
This leads of course to an error, because the
Minimize call does not see the
x, but only its value. The same happens when you try to derive this with
Integrate expressions. Although the error message is very clear, most people get very confused and try to change their whole calculation.
Exactly for those situations, the formal parameter characters are made. They all share the attribute
Protected which states that you cannot simply assign a value to them. Therefore, you can use them in situations where you maybe derive expressions or proof formulas or where you minimize an expression