I have never done the Plan Strong or the BuiltStrong. But with the information in the various articles and threads, I would say that Plan Strong is more about getting strong: 50% of the total lifts are in ~65% weight, ~35% in the 75% weight and about ~14 in the 85% weight and about ~1% in the 90/92%. Where, I think, you will do all the different weights for every training.
For this build Strong there is no 90/92% and is more about heavy quality reps. In the
podcast with Jay Ferrugia, Fabio asks him what weight he could use for a 6RM Bench. Jay responds with: I don't do benches anymore, but with a DB Chest Press around 110's per hand. Fabio then asks him how many perfect reps he could do, after a all out 6RM set, with just 2 minutes rest. Jay responds with about 3, and after those 3 I could do probably one more with 2 minutes rest again. Fabio asks how many of those 10 total reps would be perfect quality, because all of them are to failure, respons, not that many.
Now if you take the same 6RM and you will do only 3 reps the first set, 50% of you RM, take 2 minutes rest, can you do another set of 3 after this set? 'Yes'. How many would have perfect technique? 'All of them'. How many sets of these would you think you can do? 'About 5'.
So you would do 15 quality reps with a 6 RM weight (~87% of maximum weight) which will build strength, and
via Chris Beardsley also 15 stimulating reps which will build muscle aswell. With the 3 different weight categories,
Brad Schoefeld's re-examination of the Repetition continuum, also comes in to play: stimulating different fibers to work hard.