The specific contact found when a slide movement is performed with Rigidbody2D.Slide.
When a slide along a surface occurs, the slide may hit a surface tangent to the movement. This RaycastHit2D represents that contact.
It is possible that movement along a surface occurs without any contact to a surface. In this case, the RaycastHit2D will be invalid.
NOTE: This contact does not represent the surface itself that the movement is tangent to. That is represented by Rigidbody2D.SlideResults.surfaceHit.
Additional resources: Rigidbody2D.SlideResults.surfaceHit, Rigidbody2D.Slide and SlideMovement.