At least some content in this article is derived from information featured in Fantastic Beasts and Where to Find Them (film). As such, spoilers will be present within the article.
Director of Magical Security is the chief security officer for the protection and concealment of the wizarding community of the United States of America. It is a high-ranking position within the Magical Congress of the United States of America, roughly equivalent of the No-Maj post of Secretary of Homeland Security.
The Auror Divisions, the Federal Bureau of Covert Vigilance and No-Maj Obliviation, the Department of No-Maj Misinformation and the Surveillance Wizarding Resources Department all preumably fell under the purview of the Department for Magical Security and by extension, its Director, as all of the aforementioned departments in various ways were charged with keeping the magical community of America safe and hidden, both vital for the continued security of the magical citizens of the country.
The Wand Permit Office would also to some extent be answerable to the holder of said office, seen as how MACUSA could use wand permits to disclose the identity of criminals, which would also play a part in keeping the wizarding society in America safe from such individuals.
He also held the position of Head of the Department of Magical Law Enforcement under President Seraphina Picquery, in which capacity he was described as the President's "right-hand man". Percival presumably had even many more duties on his plate than safeguarding the community in this manner, though it seemed to be considered his most important duty.
By December 1926, Percival Graves was being impersonated by Gellert Grindelwald who acted in the capacity of Director to try and protect the wizarding community from the damage caused by the creatures that escaped Newton Scamander's suitcase. He also used the position to try and find an Obscurial. He acted as Percival to befriend Credence Barebone who he believed could lead him to the Obscurial until he discovered that it was in fact Credence.
Notes and references
|Magical Congress of the United States of America|