Convergence criterion: ValueTracer


I’m new to Palabos and C++ language and I’ve just started slightly modifying the permeability tutorial code to use it for my research. I have a question about the convergence criterion.
It uses three input parameters 1. velocity, 2. size, and 3. threshold. But what’s the role of velocity and size here? It says those are used to get the length of the time average and I don’t understand it. Can anyone explain the meaning of this? Tutorial uses 1 and 1000 for velocity and size, respectively, but should I modify this if I use different size of the domain? Geometry of my 2D domain is 3400 by 1062 with 5 um resolution.

Thanks so much