In his professional capacity his qualifications were high and he was judged by his peers a skillful herald, but he was a constant transgressor. As York Herald, he confirmed arms under his own seal, thus usurping the prerogative of the Kings of Arms.
The passage describes a herald who possessed high qualifications but engaged in actions that violated the established norms of his profession, specifically by issuing arms under his own seal instead of following the proper procedures set by the King's armory.