For CGMS, the available bit is always 1 when CGMS data is present, CGMS data has changed, CGMS data has not changed and CGMS is enabled.
For Closed Captioning the available bit is 1 when closed captioning is enabled, when closed caption data is detected (changed and not changed).
For VBI System 2, CCAPD bit is 1 when closed caption data is detected.