Medical billing modifiers provide additional information about a healthcare service without changing the basic procedure code. In behavioral health billing, modifier HM indicates that the service was provided by a professional whose qualification level is below a bachelor's degree. Its use can affect how Medicaid programs and other payers process claims, identify service providers, and determine reimbursement.

Modifier HM is particularly relevant to behavioral health organizations employing paraprofessionals, behavior technicians, peer support workers, and other qualified personnel who may not hold a four-year college degree.
https://mentalhealthbilling.us..../understanding-aetna