What argument can stands before the client command in IMAP4?

Question

I'm reading RFC 3501 and there are many examples like this:

C: a001 LOGIN SMITH SESAME
S: a001 OK LOGIN completed

So, what is this a001 before the command LOGIN?

Answer 1

The command's tag, chosen by the client. IMAP permits clients to issue several commands at the same time. The server processes all, and then reports to the client that a particular command is now done.

C: a001 LOGIN SMITH SESAME

A new command starts, tagged a001.

S: a001 OK human-readable text goes here

Command a001 is now done, without errors.