When you modify a face set feature, its shape and parameters are checked to make sure they match the face set feature definition. If not, the feature becomes invalid and is displayed in red in the Structure Browser. A warning is displayed if the reference element of the face set feature becomes invalid. You can change the warning display setting. See Face Set Feature Warnings.

What will make a face set feature invalid? All modifications that add material to existing faces of the face set feature or change geometry of existing faces will make the feature invalid. Some examples of operations that will make a feature invalid are moving faces so that some faces become larger, moving all faces of a face set feature, and adding tapers, blends, or chamfers.

Which changes will not make a face set feature invalid? Modifications that just remove material from existing faces of the face set feature will not make the feature invalid. These operations include the Mill, Bore, Punch, and Subtract commands found in the Machining menu.

Whenever a face set feature is invalid the part itself is still valid but it may indicate a problem for manufacturing. To make sure that the shapes of the face set features are as intended, you should modify the face set feature and select Redefine or Restore to make it valid before releasing the part for manufacturing.