When your computer talks to a peer computer (or even a "remote" server), and then receives a response from that peer or remote computer, it is not enough to know WHO that response that came from. You also need to know which conversation is being responded to.
Over time, and at any given time, you may have multiple conversations open between your computer and another peer computer (or more than one computer). You may have requested a variety of information in a mix of the same or different protocols. Some requests may take (much) longer to respond to than others. The responses to those requests are asynchronous, and will not be received in the same or in any predictable order. Some requests may receive multiple responses.
This all needs to be tracked so when a response is received, it can be identified as to which "conversation" it belongs to.
I couldn't find a specific reference to this for the case you described, but it is similar to how IMAP expects a client to provide a "tag" that is typically incremented with each new "command". For this, and I suspect in the case you are asking about, there is no requirement to "increment" the tag with each use (and so, no references are likely to be found). The only requirement is that the tag be unique for each use. Starting with a unique string (or number), and then incrementing it with each use insures it is unique, without having to specifically remember which "tags" have already been used. (IMAP RFC 3501:Section 2.2.1).