Ported vacuum has the vacuum inlet above the throttle plate, so with the throttle closed it sees no vacuum
Manifold vacuum has the vacuum inlet below the throttle plate, so with the throttle closed it sees full vacuum.
As the throttle plate opens ported and manifold vacuum are the same.
Full open throttle, ported and manifold vacuum will be near zero.
Manifold vacuum is used to add ignition advance at idle. Adding advance at idle is helpful to stabilize idle quality and prevent misfires at idle and engines that have cams with overlap that results in reversion and exhaust gases getting back into the intake charge. That exhaust and fresh charge is more difficult to light off and burns slower when lit, so on some engines and some cams, it is useful to add advance to the ignition at idle.
Off idle, as the throttle is opened ported and manifold vacuum are the exact same.
