Md. Code, Business Regulation § 9A–501

(a) Except as otherwise provided in this title, an individual may not provide, attempt to provide, or offer to provide heating, ventilation, air-conditioning, or refrigeration services unless issued a master license by the Board.

(b) Except as otherwise provided in this title, an individual may not assist, attempt to assist, or offer to assist in providing heating, ventilation, air-conditioning, or refrigeration services unless issued a master restricted license by the Board.

(c) Except as otherwise provided in this title, an individual may not exclusively maintain or repair heating systems, cooling systems, refrigeration systems, ventilation systems, or hydronic systems in the State unless issued a limited license by the Board.