Accuracy determines the length of the smallest edge Pro will recognize in your model, or the largest gap between 2 edges that will still be considered joined. Relative accuracy is a ratio between the smallest edge in your model and the longest dimension across your model. Absolute accuracy is a specific dimension for the smallest permissable edge (or the largest permissable gap)in a model.
If a part is 20" long with relative accuracy of .001, the smallest edge Pro will recognize without geometry check errors is .02". If the same parthas absolute accuracy of .001, the smallest edge recognized is .001". Hope that clears it up.