The very ethos of ATC service provision is the formulation of the contract between controller and pilot.
Now here is a rather British thing. Because the UK does this ATSOCAS stuff, we have had to invent odd concepts like the contract.
For the benefit of any jolly foreigners who are not familiar with it, it goes something like this - the pilot asks for one of the range of services available, and the controller agrees to provide it. Or the controller negotiates some other service which the pilot can agree to accept. When both parties have agreed on a particular service a contract exists and that service is provided by ATC until some other form of service is agreed. Of course, with a contract, both parties usually have to contribute and it is thus with the ATSOCAS contract - the pilot has to do his or her bit too. And this seems to be the subject of many of these posts - the pilot often appears not to know what he or she is expected to do. I guess it's a bit like the rules of cricket.
Elsewhere things tend to be much simpler in the main. It goes more like if you're inside CAS you get ATC, if you're outside CAS you don't. I guess there's still a contract of sorts but it seems that most people are clearer about their responsibilities.
But after March 12, everything will get easier in the UK. From then all the services that the pilot can get outside CAS will be called FIS...even the one that is really ATC!