Complex sentence: Thus, visibilities are only reported at a maximum of 1/4 mile increments, with visibility significantly below 1/4 mile being reported as "M1/4" (less than 1/4 mile); visibilities above 10 miles are reported as equal to 10 miles.
Split into simpler sentences: Thus, visibilities are only reported in 1/4 mile increments, with visibility significantly below 1/4 mile being reported as "M1/4" (less than 1/4 mile). Additionally, visibilities above 10 miles are reported as equal to 10 miles.