Air handlers and furnaces aren't usually used side by side in Alliance. If you use a furnace, you most likely shouldn’t need to consider an air handler.
Air handlers are usually used with heat pumps and manage airflow throughout your residence. Some models also offer secondary heating and cooling pieces to help the heat pump.
A furnace works in a different way by blowing warm air into your vents, which send it around your home. As furnaces rely on combustion chambers to produce warmth, they don't need some of the parts you'll see in a standard air handler.