Ad unit quality is defined by our bot on base of several criteria: site content, length of domain name, errors in html code, rating on Alexa (https://www.alexa.com/), etc. High quality ad units are those placed on the sites, that our bot liked best.
Newly created ad units have 0.25 quality by default. After moderation the bot defines their quality. Most of moderated sites have quality 0.4.
Be careful changing quality range of ad units for your campaign! Even a miserable change of quality range can cause dramatic change of available ad units. E.g. default range is 0.25 - 1 and you get 9k ad units. You set 0.5 - 1 range and get only 300 ad units.