Prunes are a special variety of plum that is ripe with 30% Sugar, so a Purne is a type of plum that is deyhderated. You cannot dehydrate a regular plum and get a prune. It would be a dehydrated plum. It is easier to call them Prunes since most people do not care. To be completely correct, ours were French Plums. I do not remember the scientific name any more. Prunes are littler than plums too.
Sunsweet is trying to call prunes plums as a marketing ploy but trust me, they taste very different. I like prunes when they are still on the tree and are about 20% sugar(yes, we had to measure the sugar content before we harvested them).
If you can get the farmers to give you some dried prunes, you will be amazed at how different they are from the ones they sell at the store.
Ron
Yes it is a marketing ploy. Sunswwet started chaging it a few yrs ago. They want more people to eat prunes. and those flavored prunes YUCK!!! we have plum tree and prune trees totally different fruits with different flavors. and being a 3 generation prune farmer i will always call them prunes. no matter what sunsweet wants to call them to make more money. sunsweet should have plenty of $$ we sold to Valley View up until they went out of busniess.