The relative extrema of a function (if any) occur at critical points. But this does not mean that relative extrema occur at every critical point (they could be
Can a cusp be an absolute max?
There could possibly be some x = b such that f(b) > f(a) on your interval in which case, your cusp is not a absolute/global maximum.
Can a relative maximum be an absolute maximum?
It is completely possible for a function to not have a relative maximum and/or a relative minimum. ... As this example has shown there can only be a single absolute maximum or absolute minimum value, but they can occur at more than one place in the domain.