Jamie, one could think that "extra" means more concentrated (and it may be the case) however knowing some of the TFA flavors, I am afraid that the "extra" means something totally different.
However, the good news is...I just searched in the TFA website, the only flavor they manufacture is the "extra". I believe that, if you have a flavor named just "Brown Sugar", it could have been renamed and rebottled by a revendor. And, if you are trying to replicate a recipe that asks for "Brown Sugar" flavor, the creator of it may have missed that "extra" word. Good luck!