When a given a rotationally symmetric feature can potentially be machined with a given turning axis (see Turning Axes), geometry extraction creates an IS_ABOUT relation between the feature and the axis. IS_ABOUT relations are sometimes created for partially turnable features, that is, for surfaces whose finished area is greater than the sweep area.

The site variable maxIsAboutForPlaneObstructionPercent (5% in starting point deployment data) specifies the maximum allowable non-turnable percentage of the finished area. If more than this percent of the finished area cannot be turned using a given axis, geometry extraction does not create an IS_ABOUT relation for that surface and axis.