The key was the left-hand version of the key; for example, the left-hand Control key was pressed on a standard 101 key US keyboard.
The key was on the numeric keypad, or has a virtual key code that corresponds to the numeric keypad.
NumLock key's key events indicate DOM_KEY_LOCATION_STANDARD both on Gecko and Internet Explorer. MDN,
When NumLock is locked, Gecko always returns DOM_KEY_LOCATION_NUMPAD for the keys on the numeric pad. Otherwise, when NumLock is unlocked and the keyboard actually has a numeric keypad, Gecko always returns DOM_KEY_LOCATION_NUMPAD too. On the other hand, if the keyboard doesn't have a keypad, such as on a notebook computer, some keys become Numpad only when NumLock is locked. When such keys fires key events, the location attribute value depends on the key. That is, it must not be DOM_KEY_LOCATION_NUMPAD.
The key was the right-hand version of the key; for example, the right-hand Control key is pressed on a standard 101 key US keyboard.
The key has only one version, or can't be distinguished between the left and right versions of the key, and was not pressed on the numeric keypad or a key that is considered to be part of the keypad.