Acidity in Wine
Acidity in wine is reported in terms of Titratable Acidity (TA).
Different acids affect the wine's acidity in different ways (in winemaking we're most concerned with Tartaric acid, but Malic and Citric acid can also play a role.) When TA values are calculated they are converted to Tartaric Acid Equivalents.
This conversion answers the question "What would the TA of my wine be if it only contained Tartaric acid?" This is why Tartaric, Malic and Citric acids have different effects on the TA of a wine with the same addition rate.