Consider the LinkedIn '10 of your trusted connections can introduce you to X. Please choose one:',
Let's say Bob asks Alice to introduce him to Mary. In this case a request will be sent that
- has Mary as the target identity
- has LinkedIn as the sender
- has Alice as the identity against which permissions will be performed (in order to determine whether she is allowed to proxy invites to Mary)
- on the behalf of Bob
If Bob maintained his social network somewhere other than LinkedIn, there would be another identity in the mix - that of his SNS.