Insufficient Opposing Region on Feature of Size
One characteristic of a
feature of size is that it has an axis, midplane, midline, or center point. However, an axis, midplane, 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 midplane 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.
Feature of Size Type

Requirement

Slab or Slot

(Overlapping area of projections onto midplane/Area of largest plane) > FOS_OverlapFraction

Opposed Line Elements of a Tapered Slab or Slot

(Overlapping length of projections of gage lines onto midplane/Length of longest gage line) > FOS_OverlapFraction

Planar Surface with 'Size Dimension' scheme

(Overlapping area of projection of the plane onto the reference plane/Area of largest plane) <FOS_OverlapFraction

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.