Short answer: Under the ACA, only four: the ages of the employees and dependents, the employer’s geographic location, tobacco use (where allowed), and family size/plan tier. Health status, gender, and industry can’t be used.
Small-group premiums are community-rated, so insurers may adjust them only for age (up to a 3:1 band), geographic area, tobacco use (up to 1.5:1, where the state permits it), and family size, plus the plan design and metal tier chosen. They can’t price on the group’s health history, claims experience, gender, or industry. Some states compress these factors further, such as banning tobacco rating or, in New York and Vermont, age rating.