I had trouble finding the link to that marmoset's thread. Can you hook me up? I'll keep looking for it in the meantime- sounds super interesting.
Yeah, I looked before I posted that, but I couldn't find it either. I know he did the original testing of the theory over on DGR, so the original thread isn't available right now. He put together one here because of the questions he'd been receiving about it, but I'm not sure what it was titled exactly.
A quick run down:
It's commonly abbreviated as PLH, and basically the higher it is, the more stable the disc will be relative to other discs of the same mold. Both dome and PLH are determined by the cooling characteristics of the plastic blend of the run, so they usually correlate between discs of the same mold. If a high PLH disc is flat then typically looking for a flat one will give a similar PLH, though there will always be mutants from time to time. For example: Firebirds typically are flatter when the have a higher PLH (I believe anyways...I could have it backwards here, but the principle remains the same), but it is possible to find a domey Firebird with a high PLH.
I'll see if I can find a picture that demonstrates a higher PLH vs a lower PLH.