In this model, adaptive Corona displacement doesn't make sense.
Displacement adaptivity is on by default and it's useful when you have big, flat surface with some detailed surface.
For example, a brick wall.
When you have a complete curve surface, all the mesh will be subdivided in the same level.
Screen: subdivision of the displacement depends on the image resolution and the distance from the camera.
It's a little bit complicated to calculate the exact "size". But it's useful to save ram, because Corona doesn't subdivide the portion of mesh that you don't see.
Also, more far is the mesh, less subdivision will be.
World: It's independent from distance and resolution . Easier to setup (cm, mm etc...), but it subdivide the entire mesh.
For example, if you have a long road and you are in the middle of this road, Corona will subdivide also the mesh behind you.
Useful for interactive render, but it eats a lot of ram and you have to wait a lot at the beginning of the rendering.