This writ was invented by John Waltham during the reign of King Richard II of England. He served in the English government as keeper of the privy seal from 1388–1395 which gave him the power to survey the courts of chancery.
Lawyers or solicitors might use Latin legal terms such as "Subpoena" in their consultations. The reason for this is that the legal system used in Ancient Rome, who used the Latin language, had a strong influence on the legal systems of most countries in the western world.
There is a move to make legal Latin phrases more understandable and such reforms are being made in England which is why the term 'Witness summons' is now used.