HandleUtility.PlaceObjectDelegate

public delegate bool PlaceObjectDelegate(Vector2 guiPosition, out Vector3 position, out Vector3 normal);

Parameters

guiPosition	The GUI position in the SceneView. You can pass Event.current.mousePosition to this parameter in most cases.
position	Returns the nearest intersected point to a ray cast from the guiPosition into the scene.
normal	Returns the normal of the nearest intersected point to a ray cast from the mouse position into the scene.

This is the method definition for placeObjectCustomPasses.