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Flatness constrains the form of a plane. In ASME Y14.5 the tolerance zone is the space between two parallel planes, and the surface must lie between them. Flatness has no datum. A face can be flat and still be tilted relative to the rest of the part; that tilt is Perpendicularity, Parallelism, or an angle. The Flatness Tool in 3D Auto-Dimensioning evaluates one plane from the feature set, whether the plane was extracted or authored. It does not accept cylinders or other kinds.
Use the Flatness Tool to measure the flatness of a plane

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What the number means

The reported flatness is the width of the zone that contains the evaluated surface. A smaller number is a flatter face. The Viewport draws that deviation as a heatmap on the plane, and the callout carries the value and a pass or fail state against the tolerance you entered. Erosion drops surface points within a distance of the selection’s edge. Edge points from extraction are often noisier than the body of the face. A small erosion keeps the evaluation on the face you care about. It does not change the plane feature itself.

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Workflow

1

Select Flatness

Choose Flatness from the Toolbar in Dimensioning Mode, or right-click a plane and choose Add flatness control….
2

Choose a plane

Click the plane. Eligible planes highlight on hover.
3

Set erosion and tolerance

Enter an erosion distance, in millimeters, and a tolerance. Erosion can stay at zero.
4

Submit

Submit to add the control to the feature set. The heatmap and the callout appear on the plane.
Erosion and tolerance can be edited after the control exists. The heatmap and the pass or fail state update without a new extraction.

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Reading the result

Use the callout controls to move the leader. The flatness value does not change. For a very small tolerance, increase erosion slightly before you decide the face failed. The edge of an extracted plane is the usual source of a number that looks worse than the face.