Unsolicited Response - the link (on the previous slide) isn't to the car rental service. Instead, it is back to the airline service, which pushes an (unsolicited) authentication assertion to the car rental service.
Request/Response - the link is directly to the car rental service.
The car rental service issues an AuthnRequest to the airline service. The airline service responds with an authentication assertion.
Details of these two approaches are on the following pages
.