filterJ211 is simply a wrapper for the algorithm set forth by SAE-J211 (MAR95) (APPENDIX C);
This standard describes how to calculate the filter coefficients necessary for a BUTTERWORTH 4-POLE PHASELESS DIGITAL FILTER. These coefficients can then be directly applied to an input signal using the standard Matlab |filtfilt| command or with any custom filtering function that requires any number of 2nd-order sections (pole-pairs).
By default, this function will create the coefficients, and (if the Signal Processing Toolbox is installed) filter the input signals.