A deforming mesh is a mesh whose vertices' behavior has been keyframed. The skin of a skinned character, where any deformation comes from the underlying animated character rig, could be used as a deforming mesh in reactor.

Why use deforming meshes? Firstly, rigid bodies and deformable bodies can collide with a deforming mesh as if it was another reactor body. The deforming mesh won't be affected by the collision, but the rigid body/deformable body will. So, for instance, you can create deformable clothes for a deforming mesh that you can drape realistically around his body, or you can cause environmental objects to react to the deforming mesh.

You can also attach deformable bodies to deforming meshes. This allows you, for example, to attach physically-simulated hair, garments, or tentacles to a keyframed character. When the character moves, its simulated elements swirl, swing, or slither (as appropriate) along with it. This is known as secondary motion, and is an easy way to add realism to your characters without having to animate the behavior by hand.

Unlike the other reactor deformable body types, you do not need to apply a special modifier to a deforming mesh or set physical properties for it. You just have to add the shape to a Deforming Mesh Collection. This is because reactor itself does not deform the mesh's vertices during the simulation: The deforming mesh is simply controlled by its current animation.

For information about how to attach objects to deforming meshes , see Constraining Deformable Bodies.