TA encompasses both carbonate alkalinity as well as borate alkalinity (and cyanurate alkalinity). They are all species that buffer the pool water against changes in hydrogen ion concentration. They also have different pKa values and so their buffering strength is different at different pH values. Carbonate and cyanurate alkalinity tends to buffer against increases in hydrogen ion concentration (lowering of pH) while borates buffer against decreasing hydrogen ion concentration (rising pH).
Chem geek has posted about the buffering capacity of pool water in these two posts -