L is for longitudinal, T is for transverse. L-scan is made through the sample, from outside the sample, then through the front surface, through the bulk and through the back surface. T-scan is made in a transverse direction, across the aperture. It could be made across any surface or in the bulk. Most T-scans are made across the front surface.
On L-scan, with the transmission detector and transparent sample, you see spikes from surfaces and a plateau of the bulk absorption between them. Sometimes L-scans are made only through the front surface, simply to see at which position the front sutrace is. For the reflected probe L-scans are only made to detect the position of the front surface.
T-scan helps to see inhomogeneities of coatings or the bulk absorption. While L-scan could accidentally cross the sample at an anomalous point, T-scan will give a better idea what is the regular absorption, if it is homogeneous, how many absorption spikes the sample has.