Testing to see how FP+ only sides work as far as capacity (how many people DON'T show up), which if it shows too many people aren't showing up, they may determine they need to mix some of the stand-by guests in with the FP+ track(s).
It's really just all about how the tracks are going to be broken down from what i've gathered. It could be 2 FP+, 1 stand-by, 1 FP+, 2 Stand-by or there could be a mix. That's what this test is aiming to figure out.