When you measure a plane, our software best fits all the captured points and creates a perfect plane that tries to include as many captured points as possible. There will always be points on top and below the plane that just don't fall on it. Our software takes the captured points from the top and bottom of the perfect plane and calculates the difference between the 2 as the total flatness.
In other words, flatness is the difference between the highest and lowest point on a plane after best fitting all points to a perfect plane.
Additional information on this subject can be viewed by clicking the attachment below: CAM2® GD&T Functional Explanation (Circularity, Cylindricity, Flatness & Straightness)
Click here to download: CAM2 GD&T Functional Explanation
Circularity, Cylindricity, Flatness, Straightness, GD&T, Geometric Dimensioning and Tolerancing, form