returns X map coordinate of whichUnit (alive or dead). Returns 0.0 if unit was removed or is null.

Quote from: Lep's JassdocBug: If the unit is loaded into a zeppelin this will not return the position of the zeppelin but the last position of the unit before it was loaded into the zeppelin.

Since unit extends from [jassdoc]widget[/jassdoc], you can use widget-related functions too.
See: [jassdoc]GetUnitY[/jassdoc], [jassdoc]BlzGetLocalUnitZ[/jassdoc], [jassdoc]BlzGetUnitZ[/jassdoc], [jassdoc]GetWidgetX[/jassdoc], [jassdoc]GetWidgetY[/jassdoc]