MSA or Minimum Sector Altitude should have the same meaning everywhere I believe as it is minimum obstacle (not terrain) clearance. You'll probably find that in the areas that it is used in transition are more remote areas that don't have an IF or IAF. You might also recall that you could sometimes be below this altitude on radar vectors, in which case if one loses radio contact, one will immediately have to climb to MSA.
You should have reception of the navaid to which you are tracking (in the center of the circle), but not necessarily to all other required for shooting the approach. Therefore, if no transition and no vectors, you'll have to fly to the fix in the center (normally the beacon) do the PT, and the rest of the approach.
However, you'll find that in some cases MSA is below PT or some other areas of the approach, in which case one would be wise to maintain the higher altitude (probably for reception, noise abatement,...). Keep in mind that MSA is just that and only keeps you away from obstacles and more importantly is not for approach only as it CAN ALSO BE USED FOR DEPARTURE. As a matter of fact, folks often include that in DEP briefings for emergency purposes. Hope this helps.