In real AN/ARC-164 radio, A is used to enter the active mode, i.e., anti-jamming, frequency hopping mode. T is used to receive Time-of-day, to sync the time, which is used for active mode.
In DCS, T acts as 1, enabling to use the frequencies in 1xx MHz range, which can't be used on real radio (UHF only, 255-399MHz). I don't think A has any function.