NumberFormat is an option for NumberForm and related functions which specifies how the mantissa, base and exponent should be assembled into a final print form.
With the setting NumberFormat -> f, the function f is supplied with three arguments: the mantissa, base and exponent of each number to be printed.
The arguments are all given as strings.
When no exponent is to be printed, the third argument is given as "".
The function f must return the final format for the number.
See The Mathematica Book: Section 2.8.7.
See also: ExponentFunction.