When you look at the Collision settings in Physics Properties, you’ll find Thickness Outer and Inner. I understand that Outer is an option to detect external objects like Cloth and SoftBody.
However, I'm puzzled about the purpose of Inner. Since it’s already preventing collisions from the external objects, why does Inner exist? And what happens when it scales up, and down?
Thickness outer and inner do not refer to external objects. They refer to the space external or internal to the mesh, as determined by its normals.
Here we are, a few frames into a soft body simulation, with "face orientation" display enabled:
The small soft body plane has settled into a position about 1 unit above the larger plane (as indicated by the default scale cube next door, 2 units cubed.) The outer collision margin indicates the margin, as measured in the directions of the collider's normals. Let's flip the normals and check again:
The plane falls normally, until it enters a space within 0.5 units of the collider, 0.5 units being what I specified as the inner margin....
Then is rapidly sucked through the collider-- in the direction of the collider's normals-- until it reaches 1.0 units outside of the collider, after which, it is thereafter accelerated only by gravity.
Let's put the plane above the scaling cube, give that cube collision with those same settings, reverse its normals, and see what happens:
The plane falls under gravity until it reaches 0.5 units of the top of the cube, after which it is rapidly propelled into the middle of the cube and falls under gravity until it reaches 1.0 units of the bottom the cube, after which it settles as if the bottom, reversed normal side of the cube is a floor-- settles at 1.0 units above that floor.
Inside and outside, in the context of physics, are measured by the directions of normals. The inside margin is how far it checks collision in the direction of the negative normal; the outside margin is how far it checks collision in the direction of the positive normal.
There is no need to check external objects. Collision doesn't react to anything. Any collision object is the external object, external to the sim, and all you have to do to get a mesh to interact with cloth or soft body physics is to give it a collision modifier.
