Neither of the two. It is in units of the characteristic speed in a cell, which can e.g. be the sound speed if REGULARIZE MESH CM DRIFT USE SOUNDSPEED is used. It is advisable to only have sub-sonic speeds there, i.e. < 1. The examples use 0.5. The exact mathematical description can be found in https://arxiv.org/pdf/1909.04667.pdf, section 5.1