The Combo Driver has a single coil driver and an electronic relay build into the same unit. This save money and space. It is intended for smaller engines with distributers. The relay is a solid state relay and does not have contacts that can burn or corrode.
The Coil Driver is designed for coils without electronic drivers built into them. It is only recently that new generation engines have built in coil drivers. Some cars today still requires external drivers to determine and control the charge current for the coils. New generation coils also bolt onto the plug eliminating the need for high tension leads. Most manufacturers prefer to minimize electrical interference.
The Mercury 2 ECU can drive coils with built in drivers directly but if you have to use the external Mercury coil driver for basic coils. This driver com in single channel driver configuration for engines with distributors. See the wiring diagrams on these connections.
The Single coil driver use a specialized component that can drive the primary up to 360 Volts creating a 36 Kilo Volt output on most coils. The driver is current limited to 10 Amp and has a soft discharge feature should the ECU not trigger it within a certain time. It has over temperature protection that will prevent overheating of the driver in long charge time duty cycles.
The driver requires a positive signal to charge and a negative transition to fire the coil.
The Electronic Relay is designed to eliminate mechanical parts and contacts found in relays. It is more durable and has current limit protection of 45 Amp. This relay will be switched on or off by the ECU and let battery power through to coils, injectors, fuel pump etc.
Note that a fuse box after this relay is still important to protect certain circuits from over current. 45 Amp can melt thinner wires causing a fire hazard.