The password similarity blocking feature determines the minimum changes required between the old and new passwords.

The Minimum Required Distance option allows you to define the minimum number of required differences. Distance refers to the number of character additions, substitutions, or deletions required to make the two passwords identical. 

For example, if the original password was "Flatirons2018!" and the new password was "Flatirons!2019$", the distance would be 3.

To enable password similarity blocking, go to Monitoring Policies > Password Policies and select "Password Similarity Blocking," and select a desired "Minimum Required Distance."

An option also exists to "Normalize Password First."  When enabled, before comparison, the passwords are converted to lower case characters, and common leet speak substitutions removed (o for 0, i for 1, e for 3, a for 4, etc.) 

For example, without normalization, the two passwords 'georgie' and 'G30rg13' would have a distance of 5. After normalization, the passwords would be considered identical.

To determine why a particular password fails policy, use the Test Page.