What is the difference between Messages Sent & Physical Messages Sent columns in the reports? (4333693)

What is the difference between Messages Sent & Physical Messages Sent columns in the reports?

Summary:
Provide mailbox usage data detailing the users activities. These reports specifically answer the question, How many physical e-mail messages did my users receive in their mailboxes, and how many new messages did those users create? This report does not indicate whether the message source is internal or external, nor does it record or specify the number of intended recipients.

Internal vs External
Provides a day-to-day account of sent and received messages for a mailbox, but includes the following advanced details: (1) Number of recipients for a sent message. (2) Details of internal message traffic. (3) Details of external message traffic. (4) External traffic is characterized as traffic that is external to the given Exchange organization.

Physical messages refer to the messages reported in the Mailbox-Summary report. These messages most accurately represent what the user is doing, specifically these map closest to users pressing send in Outlook. The report totals the number of physical messages a user creates and sends (or reads) without regards to the number of recipients on the To line..

The Mailbox-Internal vs External report, on the other hand, looks at the same data but it decomposes the number of recipients on the To: line. This allows the ability to determine the number of messages that were sent internally or externally.

Example, a message is sent to a Local User (the same Exchange server), a remote user (a mailbox on a separate Exchange server) and a user on the Internet (e-mail@hotmail). This is considered one physical message sent on the Summary Report but three logical messages sent (2 internal and 1 external) on the Mailboxes->Internal vs External report.