For some prefabs, I cant see the Prefab:Select , Revert and Apply options in the Inspector window. Instead of that , i see Model : Select , Revert and Open.
What is the difference between the two ?
Probably that's because you are not looking at a prefab but a model. This can be confusing, as the icons representing models and prefabs look very similar. The prefab icon is a blue cube and the model icon is the same blue cube but with a white rectangle in front of the cube.
Not sure about the exact difference between the two of them, but basically models are what is created by the fbx importer and contain only meshes/materials. Prefabs on the other hands are "blueprints" for a whole hierarchy of GameObjects and Components, that can be instantiated.