TMacTagBits(X) definition in Computer Security terms:
Definition(s): 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.
Source(s): NIST SP 800-56B Rev. 1
reference: CSRC Glossary