What radiator cap?

I have seen reference to replacing the radiator cap to resolve some heater problems. The only cap I have seen on my 99 Taurus is the pressurized cap for the reservoir. Is this the cap to which some of these threads refer or am I missing something? If there is a cap at the top of the radiator, how do you get access to it?

this is the only filler cap to the cooling system, there is no "radiator cap"