The "Reference Level" is an arbitrary point determined by Onkyo - I have no idea why they chose 62, but it doesn't matter. The actual output level depends on other factors including the input signal strength (voltage), recording level on the media, and the sensitivity/efficiency of your speakers. On my Yamaha receiver, I listen at roughly -20 to -12 dB, though it extends to + 15 or + 18 dB as I recall with "0" being reference for some reason.
Those numbers usually only mean something to the engineers who selected them, wouldn't worry to much about this. Listen to whatever volume you're comfortable with.