This filter is found in
For every pixel of the image, this filter holds the channel with the maximal / minimal intensity.
Hold the maximal channels: For every pixel, the filter keeps intensity of the RGB color channel which has the maximal intensity and reduces other both to zero. For example: 220, 158, 175 max--> 220, 0, 0. If two channels have same intensity, both are held: 210, 54, 210 max--> 210, 0, 210.
Hold the minimal channels: For every pixel, the filter keeps intensity of the RGB color channel which has the minimal intensity and reduce both others to zero. For example: 220, 158, 175 min--> 0, 158, 0. If two minimal channels have same intensity, both are held: 210, 54, 54 min--> 0, 54, 54.
Grey levels are not changed since light intensity is the same in all three channels.