There are two different descriptions in AN2586 and AN1709.
In AN2586 it says it must be left highZ, and as far as I can tell from the figure it means floating.
In AN1709 it says it should be tied to ground.
Does the two means conflict? I think highZ can only be determined by the chip itself and there is nothing I can do with it. So it doesn't matter where it's tied to according to AN2586.
So it's best tied to ground?