After the office of the *privy seal went 'out of court' in the 14c, the signet-seal evolved, allowing the king to continue sending instructions. In this period the seal's keeper was always a *clerk, never a bishop, with up to 10 clerks under him. (By the 16c, its keeper came to be called the 'secretary of state'.) The king's more personal letters were written in this office, as were his instructions to the privy seal to issue letters or warrants, including warrants to Chancery.
Dictionary of Medieval Terms and Phrases. Christopher Coredon with Ann Williams.