I'm beginning to study Analysis but I'm having the wording of this particular paragraph is confusing to me:

"The definition of boundary point (when this is not an isolated point) implies that any ball about such a point actually contains infinitely many points from S. Thus, a cluster point can be either a boundary point or an interior point of a set S but never an exterior point of S. If, for instance S is the set of rational numbers contained in the unit interval,

, then no point in S is an interior point of S and

"

It's the last sentence that loses me. Are they saying that no point in S is an interior point of S and an interior point of

. Or are they saying that no point in S is an interior point of S... AND...

?