These shortcuts are related to "Show Labeling" described in this article.
The labeling task itself defines the shortcuts associated with each label. These examples are the shortcuts for the default labeling task.
|j||Next image cropping within the Event|
|k||Previous image cropping within the Event|
|x||Toggle the visibility of the labeling panel (helpful when it overlaps with the bounding box of the object you're labeling)|
The labeling shortcut keys appear as superscript letters to the right of the labeling button:
To remove/undo an incorrect or unintended label, press the same key again. The buttons/keys act as toggle on/off. For example, if you had pressed 'v' for vehicle and didn't want to say that there was no vehicle, then you press 'v' again to remove the vehicle label.
You can change which labeling tasks are presented by specifying the task query string parameter.