Does anyone remember when champagne produced in California was formally classified as ‘sparkling wine’? ‘Champagne’ is a technical classification for that which originates from the region of France for which it is named. That makes sense. The technical classifications of prune and plum formerly made sense also, even if not universally understood. Reclassification in 2001 ruined that.
Prune, Prunus domestica, is primarily a European freestone fruit. (The pits of freestone fruits separate from the ripening flesh.) They have firmer flesh than plum, so are more practical for canning and drying. They also have higher sugar content, so might be dried without sulfuring (which prevents molding). Darkly purple and rather oblong fresh prunes are less popular than dried prunes are.
Plum is primarily a Japanese cling fruit. (The pits of cling fruits remain firmly adhered to ripening flesh.) They are softer and juicier than prune, and contain less sugar, so are not as efficiently pitted and dried without sulfuring, or canned. Larger, rounder, more colorful and more richly flavored plums are instead best fresh. They might be bluish purple, purple, red, ruddy orange, yellow or green.
Nowadays, all prunes and plums are known collectively as plums. Dried prunes are just dried plums.