Generic wildcard classes

I have this piece of code:

public interface ISet<E> {
        ...
      public boolean subSet(ISet<? extends E> other){
             for (int i = 0; i < other.size(); i++) {
                if (!isIn(other.get(i))) {
                      return false;
                }
             }
             return true;
}

Someone can tell me what would be the consequence of having used the isSubSet method as a signature:

public boolean subSet(ISet<E> other)

And if you could illustrate the differences between them.