How to turn "on" or "off" the reference levels?
In order to manipulate the level display of a reference, first open the Level Display tool and make sure the Show Target Tree icon is toggled in order to see the list of references in the file. Select the reference file you wish to change and choose the levels accordingly:
Note: when the level is highlighted it means the level display is "on" and if the level is not highlighted that means that level display is "off".