One characteristic of a feature of size is that it has an axis, mid-plane, mid-line, or center point. However, an axis, mid-plane, or center point cannot be reliably established if there is not sufficient opposing geometry. For example, for two opposing planes that define a feature of size, a mid-plane can only be reliably established in the overlapping region of the planes. If the overlapping region is small compared to one or both of the planes, it may not be valid to consider the opposing planes as a feature of size.
The following table shows the requirements for this test.
| Feature of Size Type | Requirement |
| Slab or Slot | Overlapping area of projections onto mid-plane/Area of largest plane >0.1 |
| Opposed Line Elements of a Tapered Slab or Slot | Overlapping length of projections of gage lines onto mid-plane/Length of longest gage line >0.1 |
| Planar Surface with Size Dimension scheme | Overlapping area of projection of the plane onto the reference plane/Area of largest plane) <0.1 |
The following image shows an example slab feature that references the highlighted planar surfaces.
So in this example:
Overlapping area = 10 x Depth Area of largest plane = 120 x Depth
Therefore the overlapping Area/Area of the largest plane = 10 / 120 = 0.0833
This message indicates that the opposing planes in this example should not be considered together as a slab feature.