audio samples are standard. Well perhaps this has come from SD-SDI television standards where standard AES audio packets are limited to 20 bit audio. Extended Data Packets can follow standard 20bit audio packets in SD-SDI - these contain an extra four AES auxiliary bits which can be used for 24bit audio.  

 In HD-SDI (and better) Audio Data Packets are 24bit as standard. Of course, the four least significant bits will be zero padded if presented with a 20bit PCM source. 

 - Matt Avent 2013 - 

  

 Protocol section says that 20 bit