A model is a 3D doo-hickey you look at.
A scenery object is a model placed in a track.
Eg. You can have one model (a tree) placed in a track 500 times. One model, 500 scenery objects.
Tip. If the corn field is on flat ground, you can merge many models into a larger composite, thereby using fewer scenery objects.
Beware, too many models in a small concentrated area can cause lag in the game, and if taken to an extreme it can actually cause an error.
I'm not saying don't do it. Just make sure you test to ensure there are no errors.
Also, you can place models away from the road to complex scenery level. That makes it look good for those who run the track with everything turned on, and it makes it more efficient for racers who try to get every edge they can.