The minimal viewing distance determines from what distance (m) the viewer does no longer recognize the individual LEDs but experiences the display as a homogenous area. As a guideline: the minimal viewing distance in meters is proportional to the pixel pitch in mm.
The difference to the minimal viewing distance is that the maximal distance in meters determines how big an LED wall should be approximately, so that one can make something out from afar. The guideline says that the viewing distance maximum to an LED wall per square meter increases by the factor 10. This means: the LED wall should with a distance of 100m be at least 10m2, so one can still make out details.