FXAA Parameter Confusion

I have a question about the popular FXAA shaders that are out there written using HLSL or GLSL, such as:
github.com/mattdesl/glsl-fxaa/blob/master/fxaa.glsl

Most of them mainly feature these three parameters:

I looked all over the Internet, but could not find a proper explanation about the function of these three parameters. Can anyone help?

the inquired algorithm performs 1D sampling on direction perpendicular to luminance gradient with varying range with following behavior:

reduce sample range by nearby sum with scale factor defined by

FXAA_REDUCE_MUL: reduce scale multiplier
and
FXAA_REDUCE_MIN : minimum reduce scale

after that, FXAA_SPAN_MAX will force clamp the sample range.

as the out come, the sampling vector (A, B ) will be characterize as (A', B') with min(A', B') < 1, and if |A - B| < scale factor, max(A', B') will also be less then 1, that == no blurring.