What would you expect the nominal size to represent?

The correct choice is the approximation of diameter, as nominal size refers to a name or label that provides a general idea of the size of a pipe, duct, or fitting rather than a precise measurement. In many cases, the nominal size does not precisely match the actual dimensions but indicates the approximate size of the material. For instance, a nominal pipe size may be defined as a common reference point (like "2 inches"), which helps in standardizing the fittings and components but might differ slightly in actual measurements depending on factors like wall thickness and manufacturing tolerances.

Understanding this concept is crucial in plumbing and HVAC applications since it aids in ensuring compatibility between different system components that are identified by their nominal sizes. This approximation allows for easier communication about the materials, facilitating design, installation, and maintenance across various projects in the field.