DCL06-A Use Symbolic values rather than numeric values when possible

Avoid the use of numerical values in code ("magic numbers") when possible. Reasons for this include, appropriately named symbolic constants make code more readable rather than checks against a specific number. For portability reasons also if a specific number needs to be changed reassigning a symbolic value is much easier than replacing a specific number in the code since each case has to be checked specifically.
Non Compliant Code:

if(age>=18)
{
        printf("Of legal voting age");
}

Compliant Code:

if(age>=VOTING_AGE)
{
        printf("Of legal voting age");
}

Risk Assessment

Mistakes regarding numeric values can cause unintended consequences if changes are not made uniformly

Source: http://www.doc.ic.ac.uk/lab/cplus/c++.rules/chap10.html