$NumberMarks gives the default value for the option NumberMarks, which specifies whether ` marks should be included in the input form representations of approximate numbers.
The default setting for $NumberMarks is Automatic.
$NumberMarks = True indicates that ` should by default be used in all approximate numbers, both machine-precision and arbitrary-precision ones.
$NumberMarks = Automatic indicates that ` should by default be used in arbitrary-precision but not machine-precision numbers.
$NumberMarks = False indicates that ` should by default never be used in outputting numbers.
See The Mathematica Book: Section 3.1.4.
See also: NumberForm, SetPrecision.