An air balance plug is the rubber stopper that you replace with the air filter. You can call them whatever you want but, the rest of us will stick to what they are really called.

Whatever we decide to call them is I suppose irrelevant as long as it defines the object in question enough to allow someone new to refillable or CISS systems to work out what we are talking about.
All you will do is confuse someone who is new which is probably what you are doing already.