As an observer I feel that a fictional course (I'm not sure of the course database's difference is between fictional and imaginary) offers a different set of challenges to a real course.
With fictional you are also the architect of the course, which raises a whole new set of challenges if you want a course to be realistic and players think this could be a real course somewhere.
Of course with a real course it is how faithful it is to the original, and I think there are degrees here.
Discounting Linkers DEMs for a moment, the basic is course routing and overall look, then I think you get into the degree of realism based on elevations and specific planting. In the past how far you could go was determined by available information. So for me there are different classifications of "real".
For example one course I would have liked to be a little more "real" is one called Timberlands, it is no secret that it is modelled on Bonville International, it has the right look, I would have liked it to use the actual course routing, even if elevations and slopes are not correct.