Assuming with the "Customize parking position", you mean the name of the button you click in the parking customization page, as the name says, it's used to jump directly to the Parking position customization.
To customize a jetway, you normally click the "Customize Jetways" button, which will call the interactive editor on a jetway. Of course, assuming the parking *does* have an editable jetway. That is, a jetway which has been added or replaced by GSX. If there's no jetway, or no jetway that can be edited, the button will go to the parking position.