public static float contactThreshold { get; set; }

Colliders placed side-by-side do not form a continuous surface which can result in unwanted contacts (known as "ghost collisions") when moving across these colliders. The main use-case here is to try to suppress these kinds of contacts.

When using a Rigidbody2D set to CollisionDetectionMode2D.Continuous, if a contact overlaps with a distance less than this threshold, it is automatically disabled as indicated in _enabled.

The threshold is not used when the Rigidbody2D is set to CollisionDetectionMode2D.Discrete or the threshold is set to zero.

NOTES: Caution is advised against changing this threshold as increasing it may cause valid contacts to be disabled resulting in collider penetration/tunnelling and reducing it may allow unwanted contacts to be left enabled. Also, there is no guarantee that all such unwanted contacts can be suppressed with the contact threshold.