CAS-> Variable() warning msg?

[DrD]

The example, a:=x^2, was created in the CAS environment.

Upon switching to the HOME environment, for any a(n), //where a(x) is used as a pseudonym for standard function notation f(x), but functionally the same//, is operated on according to the function's definition, just like a function machine, per the wikipedia 'function' definition.

In HOME a(n) returns the function value immediately, with no warning message.
In CAS first a warning is issued, then the same a(n) function value is returned.

I was wondering if there were some OTHER kind of operation that COULD take place using the function variables contents, different (and unintended), from the original definition a:=x^2? This could justify the CAS warning, which, perhaps, would not occur in the HOME environment.

I stumbled on this kind of variable usage (on the prime) the other day, and I'm just exploring its possible virtues.

Thank you for your time, Parisse. You've been extremely helpful!

-Dale-