An abstract characteristic of two or more articles, both of which meet the Acceptance Requirements relative to the part’s use as intended by the developer. Interchangeability is the primary criterion for assignment of a single Part Number to a purported specimen of some specific design.
This concept (interchangeability) was at the heart of the Industrial Revolution. Its importance cannot be over-stated.
Note that being interchangeable does not mean that two serial numbers having the same part number are identical. It only means that, within the established intended use, they can be swapped for each other. Therefore, an End User that has become accustomed to using a part in some fashion not foreseen in the development specification may be in for a rude surprise when the replacement shows up with the same old part marking on it. To some extent, this risk can (and should be!) mitigated in the Acceptance Requirements.