People say that Beethoven's "Für Elise" is in ABACA structure. What does that actually mean?
There's a D# because it sounds good!! Why shouldn't there be? There is no rule (at least that needs following) about which notes should or shouldn't be in a piece in a particular key! It's more than likely some teacher has said 'you can't use that note, cos it's not in the key'. A bit like when you're 5, you're told you can't take 7 away from 4...
The structure is a different issue, and A, B and C are sections. When a section re-occurs, its label (A or B), is stated again. Any new section needs a new name/letter.
That is in reference to the form of the piece. You would have a A section, a B section and a C section and this refers as to how you move between them and how you move from them. In a broader sense you could call this Form Analysis
ABACA structure simply means that there are 3 'repeated ideas'/sections: A, B and C.
It is the structure of the piece. Section A is played, followed by a different section B, then returning to A, to C and finishing in a cyclical structure with section A.
Under analysis, Für Elise does have this structure.