The behavior of the keyboard depends on a number of parameters that can be changed by the clients. These parameters are called controls. For example, the SlowKey control can be used to ignore short keypresses. Another control is the MouseKeys, which makes some keypresses to simulate mouse movements. The control only indicates whether this simulation is active or not; which keys produce the movement is not considered a part of the control, but is specified by attaching actions to these keys.
The above two controls are boolean: they are either active or not. The PerKeyRepeat is a control that is not boolean. Namely, it is a mask that says which keys are in autorepeat mode. According to the specification, non-boolean controls are "always active": that means that they always depends on a set of parameters (in this case, the mask), but that there is no single bit that can be used to deactivate the effects of the control completely.
Other than being boolean or non-boolean, controls also classifies as affecting the behavior of the server and affecting the behavior of the client library. The two above are server controls. Client library controls affect the translation of a keycode or a sequence of keycodes into a string (XLookupString) and event delivery.
Read more about this topic: X Keyboard Extension
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