Why is this an issue?
Duplicated string literals make the process of refactoring error-prone, since you must be sure to update all occurrences.
On the other hand, constants can be referenced from many places, but only need to be updated in a single place.
Noncompliant code example
With the default threshold of 3:
IF @x='Yes'
SELECT ...
FROM ...
WHERE field='Yes'
...
...
IF @x='Yes'
...
Compliant solution
DECLARE @Yes VARCHAR(3) = 'Yes'
IF @x=@Yes
SELECT ...
FROM ...
WHERE field=@Yes
...
...
IF @x=@Yes
...
Exceptions
To prevent generating some false-positives, literals having less than 5 characters are excluded.
