The percentage translates to the amount of light that is transmitted through the lens. The higher the number the more light that is transmitted. For example; if the VLT rating is 80% it would be a low light lens for darker conditions. If the rating is 20% it would be a lens for bright conditions.
Photochromatic 84-30% All conditions
Dark lens Blue mirror polarized 16% Partly cloudy to sunny
Dark lens Gold mirror 16.6% Partly cloudy to sunny
Dark lens Green mirror 24% Cloudy to sunny
Clear lens Red mirror 85% Night to partly cloudy