Regular FOS

Any feature (or pair of features) with a size dimension that can be derived from directly opposing surface elements, including:
One cylindrical surface (axis)
One spherical surface (center point)
A circular element (center point)
Two opposed, parallel surfaces (center plane)
Two opposed, parallel line elements (center line)

A cylindrical hole is a good example of an internal, regular feature of size:

Irregular FOS

A set of features that may contain (or be contained) by an actual mating envelope that is either a cylinder, a sphere, or a pair of parallel planes.

For example, the following “hole” is actually 16 separate surfaces, but the four surfaces nearest the center constitute a minor diameter, and can be contained by a cylindrical surface with a maximum and minimum size. Likewise, the four surfaces farthest from the center constitute a major diameter, and can be contained by a cylindrical surface, so both sets of surfaces can be considered separate, coaxial, irregular features of size.

The standard also says that an irregular feature of size can be a set of features that may contain or be contained by an actual mating envelope other than a cylinder, a sphere, or a pair of parallel planes. This twelve-sided hole isn’t really a cylindrical surface, but it does have a large size and a small size:

Likewise, an enclosed surface can be considered an irregular feature of size:

So the key to an irregular feature of size is “may be contained in an actual mating envelope.”