The crown was used by President Borusa to access the Matrix during another attempt by Omega to return to his native universe. It was used to find ways to deal with Omega and find the correct wording for the execution warrant for the Fifth Doctor. (TV: Arc of Infinity)
Pandora manifested herself through the crown when President Romana II placed it upon her head after she declared herself Imperiatrix and took it from K9 so that he could save Leela, Darkel, Narvin, her and the chapters from bombs planted in the council chambers by Antimon. (AUDIO: Imperiatrix)
The Presidents are categorised by in-universe chronological order. Loose chronological order applies for the lists, but when in doubt (as with the temporal uncertainty of the Time Wars), list them in the order in which they were first depicted as Presidents in the real world.
