From VFXPedia

For example, a rank filter with a size of 1 will sample 9 adjacent pixels for their values. The following shows some example values, sorted by value.

 0.0
 0.4
 0.4
 0.5
 0.5
 0.7
 0.7
 0.7

A blur filter with the same size would average these values together to produce a value of 3.9. A rank filter lets you choose a value from the list to select. For example, a rank of four would select the fourth item in the list, producing a value of.5

Controls Tab

This control determines the Size in pixels of the area sampled by the filter. A value of 1 will sample only the pixels immediately adjacent to the center pixel, while larger values sample from a larger area.

Small values are excellent for removing salt and pepper style noise, while larger values produce an effect similar to water color paintings.

The Rank determines which value from the sampled values will be chosen, where 0 is the lowest value and 1 is the brightest. If the value is .5, the pixel in the middle of the list will be sampled for the value.

Tips for Rank Filter (edit)

EyeonTips:Manual/Tool Reference/Filter/Rank Filter