I am lost with the namins conventions of the 101_GTFeature dataset. What is the role of the S selector ? T is explained in the standard but I don't find explanations on S. For example, for the Yemen sample, I found two files for the tile N12E044, with different S values :

N12E044_D101_S001_T001_L00_U0_R0

N12E044_D101_S002_T001_L00_U0_R0