Perhaps we can summarize it this way: Old Nostr DM (NIP-4) integrates four capabilities into a single Nostr key—it serves as an ID, an encryption key, a receiving address, and a sending address. New Nostr DM (NIP-17) integrates three capabilities into a single Nostr key—it serves as an ID, an encryption key, and a receiving address. Newest Nostr DM (NIP-17e) integrates two capabilities into a single Nostr key—it serves as an ID and a receiving address. I’m not sure whether the receiving address of NIP-17e messages is the ID key or the encrypiton key.