Problem with a list in CAS-program

[webmasterpdx]

So, let me get this straight, for home list access we can use () always, even if the index is a variable.
However, in a cas program or in cas mode, we must use [] if the index is a variable....thus we probably should use [] in cas mode always, or in a cas program.
I don't have a problem with this, just lets be clear about this and stick with it....
Is my understanding correct?

One more question....is this also true in vector/matrix indexing?