Its name is a Mediaeval Latin attempt at "bag of secrets" (which more correctly would have been "Saccus Secretorum").
[1] It originated during the reign of Edward IV with the storage of documents regarding treason and writs of attainder.
With the reign of Henry VIII, however, its character changed to concentrate more on the sovereign's most intimate and domestic matters which the Crown wished to keep from prying eyes.
The subjects of its contents included Anne Boleyn, Catherine Howard, Lady Jane Grey, Walter Raleigh, the Earl of Essex, Guy Fawkes, and other such cases.
The documents of the trial of Queen Anne, for example, had been thought long destroyed but after their removal and examination were found to be almost entirely complete.