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 table below shows the requirements for this test. The value for FOS_OverlapFraction is defined in the application options.

See Feature Descriptions in Model Based Design > Geometric Dimensioning and Tolerancing Advisor > GD&T Advisor for a complete list and description of supported features of size.

The picture below shows an example slab feature that references the highlighted planar surfaces. Let's assume that FOS_OverlapFraction is set to a value of 0.1.

So in this example:

Overlapping area = 10 x Depth

Area of largest plane = 120 x Depth

Thus,

Overlapping Area/Area of largest plane = 10 / 120 = 0.0833

Since this value is less than FOS_OverlapFraction, the 'insufficient opposing region...' message would be shown for this example. This message is an indication that the opposing planes in this example should probably not be considered together as a slab feature.

In the case of a Planar Surface, you should only use the 'Size Dimension' scheme if the opposing plane has sufficient overlapping area. Otherwise, you should consider the 'Offset Dimension' scheme.