Flow specs describe what the reservation is for, while RSVP is the underlying mechanism to signal it across the network.
RSPECs specify what requirements there are for the flow: it can be normal internet 'best effort', in which case no reservation is needed.
The 'Guaranteed' setting gives an absolutely bounded service, where the delay is promised to never go above a desired amount, and packets never dropped, provided the traffic stays within spec.
Those who want to listen to them send a corresponding RESV (short for "Reserve") message which then traces the path backwards to the sender.
This solves the problem if either the sender or the receiver crash or are shut down incorrectly without first cancelling the reservation.
The individual routers may, at their option, police the traffic to check that it conforms to the flow specs.
As a result, IntServ works on a small-scale, but as the system scales up to larger networks or the Internet, it becomes resource intensive to track of all of the reservations.