Royal Navy designs commonly changed as successive ships were built. Three Queen Elizabeth-class ships were slightly longer than the other two, but that difference did not make them a separate subclass. The Town-class examples involved new engines, different power ratings, and substantial internal rearrangement. A subclass therefore required a larger change than the small refinements expected during ordinary production.
Notable Point · 5:33-9:17
When construction changes justify a subclass
Slightly longer Queen Elizabeths illustrate batch changes, while the Town-class cruisers show the larger changes that justify subclasses.