SetAccuracy[expr, n] yields a version of expr in which all numbers have been set to have an accuracy of n digits.
When SetAccuracy is used to increase the accuracy of a number, the number is padded with zeros. The zeros are taken to be in base 2. In base 10, the additional digits are usually not zero.
SetAccuracy returns an arbitrary-precision number even if the number of significant digits obtained will be less than $MachinePrecision.
When expr contains machine-precision numbers, SetAccuracy[expr, n] can give results which differ from one computer system to another.
SetAccuracy will first expose any hidden extra digits in the internal binary representation of a number, and only after these are exhausted add trailing zeros.
0.004``25 generates a number with all trailing digits zero and accuracy 25 on any computer system.
SetAccuracy[expr, n] does not modify expr itself.
See The Mathematica Book: Section 3.1.5.
See also: N, Accuracy, SetPrecision.