gmx saltbr plots the distance between all combination of charged groups as a function of time. The groups are combined in different ways. A minimum distance can be given (i.e. a cut-off), such that groups that are never closer than that distance will not be plotted.
Output will be in a number of fixed filenames, min-min.xvg, plus-min.xvg and plus-plus.xvg, or files for every individual ion pair if the -sep option is selected. In this case, files are named as sb-(Resname)(Resnr)-(Atomnr). There may be many such files.