The starting zIndex that gets applied by default when an element is pressed/touched (for positional types, like "x,y", "top,left", etc. but not"rotation" or "scroll") and this number gets incremented and applied to each new element that gets pressed/touched so that the stacking order looks correct (newly pressed objects rise to the top) unless zIndexBoost:false is set in a particular Draggable's vars parameter. You can set thiszIndex to whatever you want, but 1000 is the default.
Draggable.zIndex = 500;