I believe it's done through the subject line of the email.
As an example, here's the subject line of a change approval email in my test system:
Ticket:74 Action:ApproveTicket VisibleToRequester:YES
This would break down to: update ticket number 74, approved (yes or no), are the notes visible to the person asking for the approval (yes or no)
I'm sure this can be manipulated in several ways to reassign tickets, change call-types, etc..
With that said, I'm still working out the fine details...
Anyone, please add to this or correct me if I'm wrong!!
Being able to change the client via Email subject would be a great feature. Most of us open tickets on behalf of others via email because it fast and easy.