In the MsgKit library to create a .msg invite, there is the Sender, the Recipient, and the Representing. Like the Sender, there is exactly 1 Representing. It is documented as:
These properties are examples of the address properties for the messaging user who is being represented by the MsgKit.Receiving user. They must be set by the incoming transport provider, which is also responsible for authorization or verification of the delegate. If no messaging user is being represented, these properties should be set to the e-mail address contained in the PR_RECEIVED_BY_EMAIL_ADDRESS (PidTagReceivedByEmailAddress) property.
Any idea who/what this is?
The PR_RECEIVED_BY_EMAIL_ADDRESS property contains the email address for the messaging user who receives the message. In Exchange you can use the MailItem.SentOnBehalfOfName property which allows setting the display name for the intended sender of the mail message.
When a client application sends a message on behalf of another client, it should set all the represented sender properties to the values for that client. A messaging user sending on its own behalf typically leaves the represented sender properties unset.