Usage of + or ++ in emails

I have been seeing and using the symbols + or ++ whenever a new recipient is added to an existing email thread along with the original recipients in To or Cc.

Just curious about how this came into practice. Is there a definitive understanding of how + or ++ is being used in today's email communications?

Raising this question here as am not sure if this is the right site in stackex.

UPDATE: To clarify, the + or ++ symbols are added in the body of the email, not in To or CC.

I would agree with instinct71. It's used as follows: - say an email is sent to a list of recipients, but someone was omitted or the topic needs to include other recipient(s). The new recipient(s) are added to the To: or CC: fields and their names are also added to the body of the email with a ++ or + , just to inform everyone on the current distribution that others have been added to the discussion. "++" came from C programming and non-programmers decided to reduce that to a simple "+"

There is slight difference I have observed in usage of + and ++.

• '+' Usually follows a name, to be informed or to action on.

• '++' I disagree that this is a programming jargon despite I being a programmer. ++ usually does not follow a name. This is used when more than one person has been added to email. Sometime, It may be followed by names if possible. But mostly, it is to indicated that additional peoples have been added but need to specifically mention the names is not required or it is not possible to mention the names.