  The purpose of this brief note is twofold. First, we summarize in a very
concise form the principal information on Whitney smooth families of
quasi-periodic invariant tori in various contexts of KAM theory. Our second
goal is to attract (via an informal discussion and a simple example) the
experts' attention to the peculiarities of the so-called excitation of elliptic
normal modes in the reversible context 2.
