There are two types of coolant level management system. The goal is to ensure no air remains trapped within the coolant system. Air voids in heater core degrade heater performance and make noise. Air voids in the top of the head lead to poor cooling, possible warping. Bad stuff.
In one type there is an overflow bottle, fed by a single small diameter hose attached to the fill port on the top of the radiator. The radiator cap (on the top of the radiator) has two valves.. One to blow off excess pressure (13-18 psi) and the other that works when the car cools down. In this type of system, there should be NO air anywhere within the radiator and cooling system. The overflow bottle is marked with recommended level so the system can do its job purging air during multiple drive cycles (hot, 8 hour cool down, repeat) When the vehicle is cold, and you open up the radiator cap and look it, the coolant should be at the very tippy top. Note, in my experience it generally takes about three drive cycles to purge all the air out of the system.
The other type of system has a surge bottle. Generally the top of the bottle has a pressure fill cap. There is no cap on the top of the radiator. There will be generally three hoses running to the surge tank, and the diameter of two of those hoses will be 1" or more. The bottle will be located high in the engine compartment. Because of the surge tanks high location, all the air in the system will accumulate there where it is harmless, and not remain in other parts of the coolant system. These systems self purge air from the system during normal operation, because of its height and fluid flow in Thermostat open / thermostat closed conditions. Again, recommended fluid levels are molded on the surge tank. These are important. If you are low here, its bad.