public Vector2 adjacentStartPoint { get; set; }

An EdgeCollider2D is made up of contiguous edges defined by a set of points adjacent to each other. When a collision occurs with a point along the Collider, Unity uses the two edges the point makes with its adjacent points to form a collision normal, and calculate the collision response. This produces a continuous edge chain and an uninterrupted collision surface.

However, when a collision occurs with the start or end point of the Edge Collider, Unity is unable to form a collision normal with a single edge and so the collision normal becomes the direction of motion of the collision.

This property defines a virtual point that is adjacent to the end point to create a "virtual edge" from which Unity calculates and forms a collision normal. This point is only used when _useAdjacentStartPoint it set to true. The "virtual edge" formed cannot be collided with and only a collision normal is used.

An important and useful use case for this feature is to allow multiple EdgeCollider2Ds to connect together by specifying _adjacentStartPoint and _adjacentEndPoint that overlap the points of other EdgeCollider2Ds. This produces a seamless transition for collisions when moving across multiple EdgeCollider2Ds.