The ability to deal with a single mail inside a conversation adding labels, delete or move email of a conversation without affecting all the other messages of the conversation.
E.g. if I move a single mail in a converstaion in anew folder the single mail is also moved to the folder but remains in the conversation. The same applying a label to a single mail of a conversation, without affecting the remaining messages. If i delete a mail of a conversation the deleted mail ave the deleted label but stays in the converstaion. If a delete a conversation the conversatiion is deleted and the single mail too, remaining only the ones that have been moved or labeled ina different folder.