The actual coolant system should be... as in the pressurized portion. Air in the system can cause damage to your water pump. The system will have an overflow reservoir that it can draw from or relieve to. That's what you normally check when you check your "fluid levels," and the reservoir will have high/low markings on it to let you know where the level should be.
The top of what?
You should have a reservoir chamber, usually plastic and see through, This should be full to the "Full when cold" mark when the engine is cold, (usually half way up) When hot the level may be higher.
The radiator should always be full to the top. The reservoir or the pressurized surge tank has a full line (or sometimes a hot line and a cold line) that it should be kept full to.
specify make and year. It matters.
