The DAMERAU modifier is used with the LEVENSHTEIN routine to enable scoring deduction change according to the Damerau algorithm. The Damerau algorithm allows adjacent transpositions in addition to insertions, deletions, and substitutions.
Example
String 1 - ISABELLA
String 2 - ISABELAL
When these strings are compared without the routine, the distance is 2 and the score is 99-2=97, since two edits (substitutions of 'A' for 'L' and 'L' for 'A') are required. With the DAMERAU routine, the distance is 1 and the score is 99-1=98 since adjacent transposition is allowed ('L' and 'A') and the transposition is counted as one edit.