Open schemes (Discovery, Bonitas, Momentum, Bestmed, Medshield) are available to the general public. Closed schemes (GEMS for public servants, Polmed for police, BankMed for banking employees) are restricted to specific employer groups or professions. Closed schemes often have lower contributions because the employer subsidises and the risk pool is healthier (employed people).
SA example: GEMS is the second-largest scheme in South Africa with 2.4 million beneficiaries, but only government employees can join. Discovery is the largest open scheme.