Number - The current y (vertical) position of the Draggable instance. For a Draggable of type: "x,y", it would be the y transform translation, as in the CSS transform: translateY(...). For type: "top,left", the Draggable’s y would refer to the CSS top value that’s applied. This is not the global coordinate - it is the inline CSS-related value applied to the element.
This value is updated each time the Draggable is dragged interactively and during the momentum-based tween that Draggable applies when the user releases their mouse/touch, but if you manually change (or tween) the element’s position you can force Draggable to look at the “real” value and record it to its own y property by calling the Draggable’s update() method. Basically that re-synchronizes it. Again, this is not necessary unless other code (outside Draggable) alters the target element’s position.