What is the exact formula for relative translational error?

[BahMoh]

Since the authors, did not bring in the formula for the relative translational error and the description,

Relative Translation Error (trel): which is the averaged relative translation increment error for all possible subsequences of length 100 m, . . . , 800 m, in percent of the traveled distance

is a little vague.

I found nothing in the code or in the literature. Can someone please provide a source for this formula, or describe it here?