Open Sound System
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This capability means that the audio device is hidden. It is reserved for some special purpose and should not be presented in audio device selection lists of normal applications.
For example the server side of a loopback audio device is hidden because it's usually used by server applications designed for this purpose.
Audio device capabilities like PCM_CAP_HIDDEN can be checked using the SNDCTL_AUDIOINFO or SNDCTL_DSP_GETCAPS ioctl calls.