How to use a service description in OWL-S?

Hi

At the moment I am trying to understand OWL-S and what it is good for.
....but somehow I'm lost...

First I don't completely understand the grounding of a CompositeProcess
using WSDL. In the technical overview I read that "If the composite
process has an overall effect, then the client must perform the entire
process in order to achieve that effect." So e.g. if I have a sequence
the steps have to be processed in the right order. But how is this
requirement expressed in the  wsdl document. I am trying to understand
the Bravo Air example but how can I see in the Grounding.wsdl that a
client first has to login before it can confirm the reservation? 

Another point I don't understand is, how software client are supposed to
use the Service instance.owl documents to understand how to use a
particular service. A software client easily can get out of the
BravoAirService.owl file that for finally booking a flight it first has
to login and than confirm the reservation. But other airline can have
their own owl files describing how to book a flight. But don't have all
the airlines have to agree on a concept "Flight Booking"? 

Maybe somebody can give me links for further reading.
Thx
Gerhard