In all my records, I've included in 336, 337 and 338:
$atext $b txt $2 rdacontent
$aunmediated $b n $2 rdamedia
$avolume $b nc $2 rdacarrier
but I'm not sure what the significance of the $b elements are and
whether they need to be included or not. I confess, I saw these $bs
in some example RDA MARC records.