Apologies if this has been already answered somewhere here, but I really can't understand/find what is the actual notion of conversation in Gmail.
According to Google:
Gmail groups all replies with their original message, creating a single conversation or thread.
Anyway I have seen that sending two emails with the same "Merry Xmas" subject groups them in a conversation too.
Where can I find a formal definition of conversation or, put it differently, what are the rules to group message A and B under the same conversation?