Numbers entered in the form digits`p are taken to have precision p.
Numbers such as 0``a whose overall scale cannot be determined are treated as having zero precision.
Numbers with zero precision are output in StandardForm as 0.10-a, where a is their accuracy.
If x is not a number, Precision[x] gives the minimum value of Precision for all the numbers that appear in x. MachinePrecision is considered smaller than any explicit precision.
10-a, where a is their accuracy. 
EXAMPLES
Basic Examples 
Scope 