Short answer: It’s the ACA pricing method that lets premiums vary only by age (up to 3:1), location, tobacco use (up to 1.5:1), and family size, never by health status, gender, or industry.
Modified (adjusted) community rating is the system the ACA established for the individual and small-group markets. Insurers may adjust premiums for just four factors; age, geographic area, tobacco use, and family size, and nothing else, so two businesses with the same demographics pay the same base rates regardless of employees’ health. It replaced pre-ACA medical underwriting. Some states compress it further (banning tobacco rating, or in NY and VT, age rating).