@User @User low or high Vt or in some technologies very high/low means the magnitude (low, high, very high /low respectively) of vt. In nmos it is positive, gate higher than source and in pmos gate lower than source voltage. Low vt are used for high speed applications, but the gate leakage will be higher. Also low vt has thin oxide gate oxide, hence it should not used for higher voltage uses due to reliability concerns