What Does "Reference Level" Mean?

Absolute vs. relative volume level

Many receivers have the ability to switch between an absolute volume scale, and a relative volume scale. We suggest using the relative scale, as this is more consistent across the board than the absolute scale.
The relative volume scale ranges from -79.5dB up to +18dB where 0dB is the reference level. The absolute scale can vary between manufacturers, but generally ranges from 0-98, where 70 is reference level. Some manufacturers do this differently, so please check your manual if you use the absolute scale.

Reference level

Movies are mixed to target a volume of 85dB with 20dB of headroom according to Dolby/THX standards. That means the loudest peaks will reach 105dB. When running your calibration, your receiver will play test tones that are 75dB instead of 85dB, simply becuase it's more gentle on your ears. After calibration, setting your receiver to 0dB on the relative scale will mean reference level.