Each scheme sets a 'scheme rate' for every procedure and consultation: the amount it will pay in full. Doctors are free to charge more (often 200 to 300% of scheme rate), and you owe the difference unless your plan reimburses specialists at a higher percentage of scheme rate, or you have gap cover. Discovery calls it DHR; other schemes have their own naming (BHF tariff, Medical Aid Rate, scheme tariff).
SA example: A specialist consultation: scheme rate (100% DHR) might be R900. Your specialist charges 300% = R2,700. Your plan pays at 200% DHR = R1,800. You owe R900 out-of-pocket. This is what gap cover exists to address.