When is a conversation a conversation according to Mail app?

I don't really get when Mail.app groups mails together as a conversation and when it doesn't. Example: I've written the same mail separately to multiple recipients. So, not all recipient addresses in the "To:" field for one mail, but individual mails to every single recipient. Now some of them have responded in person, some of them sent out-of-office notes, and some both (they responded once they got back in office). Now, the Mail app groups SOME of these exchanges to conversations, but not others, e.g. for some, the out-of-office reply is shown as part of the conversation, for others it isn't. Sometimes also the actual reply is not included in the conversation, but the out-of-office reply is. I thought that maybe some recipients responded from a different address that I had sent my mail to, but that isn't the case. Also, the subject doesn't seem to play a role. Sometimes the mails are not grouped even if the subject of the reply is "Re: " + the original subject, sometimes they are grouped even if the recipient replied using a different subject. What are the actual criteria by which Mail decides when to include a mail into a conversation and when not?