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NF is used with NJ in idiomatic number expressions involving "of":

          +-NW-+---NF---+ +NS+
          |    |        | |  |
He lives two thirds    of a mile from here
He lives one third     of a mile from here (uses NS instead of NW)
He died three quarters of a century ago

He lives      3/4      of a mile from here

Fractional words like "thirds" and "third" have NF+, as do 
fractional numbers like "3/4". "Of" has the complex "NF- & NJ+"
disjoined with everything else. Singular words like "mile" (not
plural forms: "*He lives two thirds of 5 miles from here") 
have an optional NJ-. 

NF and NJ are similar to ND in that they are only used in
idiomatic expressions such as time as place expressions.
Fraction words like "third" can also act as noun phrases (with
the right determiners), and thus they carry "S+ or O- or J-",
like nouns; they can also take modifying phrases such as "of"
phrases using "M-". For this, no special apparatus is needed:

        +NS-+-M--+----J---+        |
        |   |    |        |        |
	A third  of the population is illiterate
      Two thirds of the population is illiterate (NW instead of NS)

Grammar Documentation Page.