n GD&T the term feature-of-size (FOS) refers to any surface, or set of parallel surfaces associated with a size dimension. Specific examples of features of size include:
A hole diameter (a cylindrical surface)
Plate thickness (two opposed parallel surfaces)
Peg diameter (another cylindrical surface)
A ball bearing diameter (a spherical surface)
The example below shows three views of a plate with two holes. There are two dimensions called out in this drawing: a plate thickness and a hole diameter. Either of these is considered a FOS.