When an aircraft intends to make a flight, well before the flight is set to depart the dispatcher for the airline (or the pilot in a non-airline situation) files a flight plan, which is sent to each of the ATC units the aircraft will travel though. The flight plan contains, among other items, the exact route and the altitude.
When an aircraft calls clearance delivery, all they are doing is just validating the flight plan that the airline has requested and ammending anything that doesn't work for the controllers (i.e. re-routes around busy sectors etc.). For the most part though, what the airlines file in their flight plans is what they get cleared for.
99.9% of the time (at least here in Canada) all clearance delivery says to an aircraft is, for an example, "NWA123 is cleared to the KMSP airport via Winnipeg Nine departure, flight planned route, squawk 1234."