Max PCM Rate: uses the maximum possible sample rate for your device/endpoint
Max PCM Rate (Power of 2): the same as above, but it has to be a multiple of the power of 2 times the sample rate of the file to play (so 2,4,8, etc)
In your case, 44.1 kHz * 4 = 176.4 kHz. Even if your device can play eg. 192 kHz, the setting with “(power of 2)” will not use that sample rate, as it is not a power of 2 times 44.1 kHz.
Btw, you can find a lot in the Roon Knowledge Base:
Garanteed for some hours of reading 