Absolute TOF is a time measurement made in a single direction. Time Measurement Accuracy (TACC) has to do with differential time measurement which is the difference between time adjacent “up” and “down” absolute TOF’s. Much of the time error internal to the TDC is due to electrical path delays. These delays mostly cancel in the subtraction required to arrive at a delta TOF.
Absolute TOF isn’t relevant to a flow measurement application. TACC is relevant to flow measurement applications because it is the delta TOF that is used to determine flow rate. Absolute TOF is relevant to pulse-echo applications.