The measurement from mouth to genitals. Simply for establishing a sufficient partner for 69. The greater the disparity in oral orbit the less likely 69 will be pleasureful for 69ers.
Chris says " man i love when a chick sucks you off and you get to dig your tongue into her beaver."
Joel responds " yeah i know what you mean man danielle damico and i have an amazingly similar oral orbit, her beaver is always 100% in my face!"