It may help to know where MARP came from:
Originally, call redirection was controlled by the first TN in the TN list. You can see the TN list if you do a DNB lookup in LD 20. Unfortunately, in pre-MARP releases, if you performed a change on a set it changed the oder of the TN list (it does it now too, but because of MARP, it doesn't matter). So if the TN list started out as:
4 0 0 0
4 0 0 1
4 0 0 2
And all of the call redirection options were expected to come from 4 0 0 0; then you performed some sort of change on 4 0 0 1, this is what the TN list would look like:
4 0 0 1
4 0 0 0
4 0 0 2
Now, all of the call redirection options would come from 4 0 0 1. If you wanted to switch them back, you had to do a trivial change to 4 0 0 0. MARP is a way of eliminating this procedure and lock down the way the call redirection options are handled.
Hope this helps...