Employer contributions to eligible employees’ HSAs must be treated as comparable. Comparable contributions must be made to all eligible employees enrolled in a qualifying HDHP, are in the same category of employment (full-time, part-time, former employees, and collectively bargained), same category of coverage (self, self +1, self +2, self +3). In order for it to be considered comparable, an employer must contribute the same dollar amount or percentage amount to everyone in that category. Additionally, the higher coverage categories can’t have a lower contribution amount compared to the lower coverage (self +1 has to be greater than self).
Source: IRC Section 4980G and the regulations thereunder at Treas. Reg. Sec. 54.4980G