NumberMarks is an option for InputForm and related functions that specifies whether ` marks should be included in the printed forms of approximate numbers.
The default setting for NumberMarks is given by the value of $NumberMarks.
NumberMarks->True indicates that ` should be used in all approximate numbers, both machine-precision and arbitrary-precision ones.
NumberMarks -> Automatic indicates that ` should be used in arbitrary-precision but not machine-precision numbers.
NumberMarks -> False indicates that ` should never be used in outputting numbers.
Number marks are used to indicate the type of numbers, and their precision or accuracy.
The *^ form for scientific notation is always used in InputForm, and is independent of NumberMarks.
See The Mathematica Book: Section 2.11.3 and Section 3.1.4.
See also: NumberForm.