TN I-94 card is different from othe types of I-94 card (e.g. F1, H1 etc.). TN I-94 card normally is multiple entry I-94 card valid for the entire TN period no matter whether you go out of U.S. and reenter U.S. So, The TN holder is not supposed to provide the I-94 to the airlines (CBP) when he/she leaves U.S. unless he/she will not come back to work with the same employer.