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I would imagine that all it does is add to how long the message is displayed.
I do not really know what practical use this has, unless perhaps you want to put in a feature that enlengthens how long text shows
As far as I know transmissions include wait action and the next transmission wont play before the previous had finish, so what is the use to substract/add value if its previous state was 0. I definetly cant understand Blizzards