Each queued message is identified by a unique identifier (Section 11.2.1). The $i macro contains as its value that identifier. Prior to V8.6 sendmail, $i had a value assigned to it only when a file was first placed into the queue. Beginning with V8.6 sendmail, $i is also given a value when the queue file is processed.

$i is not used by sendmail internally, nor should you use it in a rule set. It should be trusted for use only in the Received: and Message-ID: headers.