I have this scene:
The pillow is with Cloth physics (pressure=5), and the ball is with Collision (not a rigid body).
But when I start the simulation, it ends up with:
As you can see there is a gap between the ball and the pillow. Where did it come from? The ball is not a Rigid Body, so there isn't sensitive margin.
If I make the ball a Rigid Body, it will simply clip through the pillow: