What is the meaning of positive f(d)?

[MichaelWangGo]

Hi authors, thanks for excellent work, but I really cannot understand the sentences in 3.1. Adaptive Group Correlation Layer, "In 1D search mode, we set g(d) = 0 and f (d) ∈ [−r, r], where r = 4. Positive displacement value of f (d) is reserved to adjust inaccurate results after every iterative sampling."