18.104.22.168.4 Degree of Difficulty
This parameter contains the degree of difficulty for which a puzzle solution is sought. A larger Degree of Difficulty value indicates that the puzzle-generating application used more computing resources to create the puzzle. Therefore, the receiving system typically assumes that a larger Degree of Difficulty value corresponds to a lower likelihood that the message is spam. This is only a generally accepted guideline, and is not a protocol requirement.
This parameter MUST be a positive integer value that is formatted as type string.<3>