I think it refers to the receiver (speaker) size for RIC type aids. Specific to the OPN, it should be 60, 85, 100 and 105dB.
When you’re talking about “within the bounded area”, you’re probably talking about the fitment. Yes, you usually want your loss to be within the fitment area. Right on the line is fine but you basically have no margin left and if your hearing gets worse you may need to upgrade to a bigger receiver. You don’t want your loss to be outside the fitment area for obvious reasons -> receiver not big enough to deliver the needed amplification -> may cause distortion due to overdriving the undersized receiver.
You don’t want to oversize the receiver either because the bigger the receiver, the higher the THD (Total Harmonic Distortion) spec may be and you want to get the one with the best THD spec possible.
Certain big size receiver like the 100 and 105dB usually require custom mold. The smaller 65 and 85dB can use the pre-built dome types.
If you have asymmetrical hearing loss then it’s possible that you may need different size receivers for each ear. You want to have the correct (volume) level for each of the ear’s hearing loss. If they’re asymmetrical then different levels for different ears.