Examples of qualities centered around number 6 that women yearn for include a six foot height (at least), a six-pack abdomen, six years of higher education (i.e. a university degree), six months since last relationship, a 6,000 sq ft house, a non-bone-pressed six inch penis (at least), a six inch penile girth, a six-hundred horse-power car, a six-figure salary, at least a 6th decile in terms of facial attractiveness. The associations of these male attributes with the number 666 derives from the shallowness and superficiality of such high standards that some associate as being equivalent in evil to the Number of the Beast (666). However, most women realize that such perfect requirements in a man will probably doom them to be a permavirgin cat lady. Therefore most women give some leeway.
Opposition to the hypergamy associated with the 666 dating rule is called 666phobia.
The 666 dating rule doesn't imply that there is a volitional effort on the part of women to choose each of these traits. Rather it is a convenient shorthand for approximately describing a largely subconscious mating choice on the part of women. Although 666 is meant to convey shallowness, it is not the most detrimental-to-society dating choice that has been observed among women. That award would go to the surprisingly common tendency of women to be attracted to men with traits such as dark triad and hybristophilia, in other words, the asshole or bad-boy archetype.