I grew up thinking prunes and plums were different names for the same fruit. We had trees that produced the oblong purple things. I didn't see a red, round plum until I was an adult. I had to look it up to discover that both the red round things and the oblong purple things are plums, but from different branches of the same genus (prunus). Only the purple things are used to make dried prunes. For this reason they're often (in the US, anyway) called prunes in their fresh form as well. Red plums remain plums and are not "suitable" for being dried. I couldn't find a reason why not. Trying drove me plum loco.