I am not sure but I have heard that while the Pandit's hat represents the mastery of Sutra, The Panchen hat represents the mastery of both Sutra and Tantra. Panchen Sonam Dragpa was one of the most illustrious masters in the Gelugpa tradition, and he still is and his texts are used for debates and studies because they describe the most profound practices in ways that people can understand, study, apply into practice and gain results. So logically and also from my own naive mind, I would say that the Panchen hat represents the complete mastery of all of Panchen Sonam Dragpa's texts and people who wear it would have.