A network plan trades flexibility for lower contributions. Use a network provider, get full cover. Use anyone outside the network, get partial cover (typically 70%) or pay the full out-of-pocket cost. Networks come in flavours: KeyCare network (entry-level), Smart network, Delta network (smaller hospital list), and Coastal/Regional variants. Every plan with 'Network', 'Smart', 'Delta' or 'KeyCare' in its name uses this trade-off.
SA example: Discovery KeyCare Plus only covers you in KeyCare hospitals. Going to a non-network private hospital means paying the difference yourself, or 30% of the hospital bill at minimum.