Experimental`$SameQTolerance

$SameQTolerance gives the number of decimal digits by which two numbers can disagree and still be considered the same according to SameQ.

The default setting is equal to Log[10, 2], corresponding to a tolerance of one binary digit.

See also: $MachineEpsilon.

Note: this is an experimental feature, and in future versions of Mathematica it may not be supported, or may have a different specification.