By numismatic convention, followed by
Cohen, the early
RIC volumes, and
BMC, a "
head" has no adornments except for a possible
wreath or
radiate crown, while a "
bust" has adornments, usually a fold of the cloak or
aegis on the front shoulder and behind the neck, or a
cuirass worn on the upper body and covering both shoulders, or a cloak pinned around the neck, or both
cuirass and cloak.
Obviously heroic nude busts including much of the upper body are exceptions and cannot be called merely "heads" because they lack adornment!
Varbanov is extremely careless so I wouldn't waste time trying to figure out a logical system behind
his terminology. I don't doubt that for the most
part he simply copied
his bust descriptions from the catalogues he was excerpting.