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This capability tells that the device is suitable to be used as a default audio device in normal audio applications (like games and media players). It is likely but not guaranteed that this device is connected to the primary speakers (headphone) of the system. For input devices this capability means that the device is connected to the preferred microphone.
Devices without this capability may (or may not) be special purpose devices and not necessarily suitable for normal audio output. For example such devices may be allocated for some dedicated purpose such as voice announcements in a large airport. Playing default system beeps to such devices may be annoying.
This capability may be managed automatically. In such case it's not possible to detect reliably which device is connected to the speakers. So applications using this capability should be prepared to handle multiple choices.
This is a new capability in OSS 4.0. Earlier OSS versions don't support it. For this reason applications using this capability must check if any of the available audio devices support this capability. If no device reports it then the application cannot depend on it.
For applications that don't have user configurable audio device selection it's recommended to use /dev/dsp as the audio device. Searching for the first audio device with PCM_CAP_DEFAULT capability is completely unnecessary and may cause serious problems in certain systems.
Audio device capabilities like PCM_CAP_DEFAULT can be checked using the SNDCTL_AUDIOINFO or SNDCTL_DSP_GETCAPS ioctl calls.