public float edgeRadius { get; set; }

The edge radius controls a radius extending around all edges of the Composite Collider. When an edge has zero radius it is effectively infinitely thin. When an edge has a radius greater than zero, each edge acts like a capsule shape with rounded ends. This results in a Composite Collider with rounded corners.

It is important to know that when using _useAutoMass, changing the edge radius does not change the calculated _mass even though the collision area has changed. The mass is calculated as if the edge radius is zero i.e. not used.