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From The PSUPedia
STA is short for Stamina. It allows a character to better resist infliction of Status Effects, similar to the ESP stat from PSO, which was at the time exclusive to monsters. This stat is exceptional in that one does not gain any additional points in it at higher Character Levels. Stamina is also higher in females than in males.
STA can be increased using:
Formula of the chance to resist status effects: unknown