Lakkhana means that mark or seal. An imperial seal certifies the authenticity of documents, allowing for passage without hindrance. Here, it is used in analogy for the principal doctrines of Buddhism. If any doctrine is in line with the “Seals of Dhamma”, it can be regarded as authentic teaching of Buddha. (If it has been mastered, all Dhammas will be grasped without obstruction.) That is why they are called the “Seals of Dhamma”. If, after “All phenomena are impermanent, all dhammas are devoid of self”, we add “Nibbana is serene”, we have what are called the Three Seals of the Dhamma. With the addition of “All defiled dhammas lead to suffering”, we have the Four Seals of Dhamma. (From Essentials of Buddhism: Questions and Answers)