Understanding material .mtl files I am reading this piece of documentation to understand mtl files

So far I am stuck trying to understand the difference between ambient, diffuse and specular reflectivity (Ka, Kd, Ks).

If I am correct specular is "straight line" reflectivity (the kind of reflectivity of mirors), diffuse is the reflectivity of scattered light (the kind of reflectivity of many rough plastics). But then what is ambient reflectivity?
@PaulHK explained it very well in the comment section:


  Ambient is a (rough) approximation of indirect lighting. For example, an outdoor object would be lit by the sun but the surfaces facing away from sun would be black if using only lambert shading, we can use dark gray for the ambient colour to mimic the effect of indirect lighting from the ground and sky


If you were to visualize this with a shading model, say Phong, the Ka, Kd and Ks values would correspond to the following lighting calculations.