Every scheme maintains medicine formularies for both chronic and acute care. The formulary lists which specific brands and generics qualify for full cover. Going off-formulary means: full cover up to the formulary equivalent's price, and you pay the difference. Many schemes have an exceptions process; if your doctor motivates that the formulary option isn't clinically suitable, the scheme can approve full cover on the alternative.
SA example: Your doctor scripts a brand-name statin at R450/month. Scheme formulary includes the generic at R180/month. You either accept substitution or pay R270/month difference.