BCDx36HP What exactly does the P25 Threshold setting do?

My experience is that setting the P25 Threshold value manually to 10 or 11 improves the x36 performance on my local Motorola (non-P25) trunked system.

So I infer that the P25 Threshold setting must affect the decoding of IMBE-coded audio, not the C4FM or CQPSK modulation or the MAC layer of P25.

But what IMBE parameter is being adjusted?

And, by the way, why isn't the automatic optimization working?