What are conversations?
Gmail organizes mail into Conversations, or groups of messages, based on the subject line and content of the email. Conversations give you the context of the email by displaying previous related messages.
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A Conversation may contain messages between a large group of people, as well as private conversations between a smaller group than the original message. When you Reply All to a message within a conversation, the people in that specific message will be the ones who receive your reply. You can determine who will receive your response by looking at the To: and CC: box on top of the email you are composing.
When you reply to any message within a conversation, nobody will see any text they have not seen before! Only if you add additional people to the To, CC, or BCC box will new content be introduced to those recipients.