It's a conceptual question.
For example, if B is on A (each has mass), and A is on a surface with no friction, then as A accelerates, how would friction work between A and B?
My guess is this: When A accelerates, initially B will accelerate with it as a single body system. However, if the force exerted on A is greater than the force of static friction between A and B, then B will begin accelerating in the opposite direction of the force accelerating A. The force accelerating B is the kinetic friction between A and B.
If the force on A is released, though, will the friction between A and B accelerate A in the opposite direction? And how will this affect the acceleration of B?