  We discuss a new perspective on the dualities among seven-dimensional
M-theory on elliptically fibered K3 surfaces, eight-dimensional (8D) heterotic
strings on $T^2$, and 8D F-theory on elliptic K3 surfaces. There are several
distinct small-fiber F-theory limits of a single M-theory, and we deduce that
the distinct F-theory limits of an identical M-theory generally include both an
F-theory limit, wherein a discrete gauge symmetry forms, and another F-theory
limit, wherein a discrete gauge symmetry does not form. We also discuss
constraints imposed on the degrees of discrete gauge groups and on the
continuous gauge groups formed in F-theory on K3 surfaces by applying a formula
that is known to hold for genus-one fibrations of K3 surfaces, and by utilizing
the existence of the Jacobian fibrations.
