Flatness is the simplest geometric control on most drawings and the most often misused. It holds every point of a single surface between two parallel planes separated by the tolerance value, and it takes no datum reference because it compares the surface only with itself. If your callout has a datum letter beside it, you wanted parallelism or perpendicularity instead.
The zone and what it does not say
The two bounding planes float: nothing fixes their orientation in space, so a plate can be flat within a very tight value and still sit at an angle to everything else on the part. That is exactly what makes flatness the right control for a gasket face, a sealing surface or a reference face that will be clamped down, and the wrong control for a face that must be square to a bore.
Flatness against parallelism and profile
Parallelism controls a surface against a datum, so it limits both the form of the surface and its orientation, and its value therefore also bounds flatness. Profile of a surface goes further and bounds the surface against its theoretical position as well. Calling out all three on one face is a common drafting habit that buys nothing and prices three inspections.
How a shop actually holds it
Milling a face in one pass on a rigid setup gets most commercial values without special measures. The problems are release of residual stress after material comes off, clamping distortion on thin parts, and heat. Tight values on a large or thin surface usually mean a rough cut, a stress relief, then a finish cut, or grinding on a surface grinder afterwards.
Measuring it honestly
A surface plate and an indicator sweep is the traditional method and is fine for generous values. A coordinate measuring machine takes a grid of points and fits the planes numerically, which is what most shops will quote for. Either way the part must be supported without being pushed flat by its own fixture, because a clamped measurement of flatness measures the clamp.
Questions people ask about gd&t flatness
Does flatness ever take a datum?
No. It is a form control and compares the surface only with itself. If the requirement involves another feature, the correct symbol is parallelism, perpendicularity, angularity or profile.
Is flatness needed if I already have a thickness tolerance?
Often not. A tight size tolerance across a plate limits how far the faces can wander relative to each other, so a separate flatness callout is only needed when the surface must be flatter than that.
What flatness is achievable on a machined face?
Ordinary milling holds values that suit sealing and mounting faces without extra work. Anything appreciably tighter usually needs grinding or lapping, and on a large plate it may need stress relieving between cuts.
Why did my flat part arrive bowed?
Usually residual stress. Material that has been rolled or heat treated releases stress as the cut removes the skin, and a part that measured flat on the machine relaxes overnight. Say so on the drawing and the shop will plan the cuts differently.