A truncation function that outputs the most significant (i.e., leftmost) MacTagBits bits of the input string, X, when the bit length of X is greater than MacTagBits; otherwise, the function outputs X. For example, T2(1011) = 10, T3(1011) = 101, and T4(1011) = 1011.
Sources:
NIST SP 800-56B Rev. 2