Syn = Synonym aka if you click that tag you will see everything it is synonymous to and not only that thing. Aka the Kal-El tag shows you stuff tagged Clark Kent, Kal-El, Superman/Clark Kent, Clark Joseph Kent, Superman, etc.
Subtag = The tag still stands on its own. You would only see fics tagged with Kon-El where Superman (pretend this is how the tag usage is reflected, here) was an alien/on Krypton. However sinxe it is subtagged to the Superman tag, if you went to the Superman tag then you would see everything else that was sub tagged to it, aka Clark Kent, Kal-El, Superman/Clark Kent, Clark Joseph Kent, Superman, etc.
In this case something like Ancient Rome RPF tag would only show that, and with subtagging the Historical RPF would show all hisortical RPF, including Ancient Rome RPF.
Re: So, how do we try to get umbrella fandom de-synning to happen?
Subtag = The tag still stands on its own. You would only see fics tagged with Kon-El where Superman (pretend this is how the tag usage is reflected, here) was an alien/on Krypton. However sinxe it is subtagged to the Superman tag, if you went to the Superman tag then you would see everything else that was sub tagged to it, aka Clark Kent, Kal-El, Superman/Clark Kent, Clark Joseph Kent, Superman, etc.
In this case something like Ancient Rome RPF tag would only show that, and with subtagging the Historical RPF would show all hisortical RPF, including Ancient Rome RPF.