Ported or manifold Vacuum to Dist.

In a relative nut shell...
Manifold vacuum will be high at closed throttle.
Ported will be high at open throttle.

Some older Ford's I have had had both going to the advance canister. And it kind of averaged the two.

The CAP valve does some thing with both to slow the timming return to no vacuum, IIRC. There is a CAP factory manual on imperial club web site again IIRC