The Parts of the Earth do not stand alone. They are one division of a larger object, the Great Table, which is the central artefact of Dee's Enochian system.
The Great Table is a grid of letters, twenty-five squares wide and twenty-seven high, delivered to Dee across several sessions in 1584. Its first division is by a cross of single-width arms, the Cross of Union or Black Cross, which separates it into four rectangular tablets, each twelve squares wide and thirteen high. These four are the Watchtowers, and they are assigned to the four traditional elements.
Each Watchtower is then divided internally by a Great Cross into four Lesser Angles, so that the elements are subdivided by element in the usual way of this kind of system. Names of God and of angels are read out of the tablets by prescribed routes through the letters: down a column here, across a row there, sometimes with a letter borrowed from the Cross of Union. The angels so derived were later given the labels Kerubic and Servient by the Golden Dawn, which is why those terms are ubiquitous in modern books and absent from Dee.
The ninety-one Parts come from a different method of division applied to the same table, one which cuts it into ninety-one dissimilar regions and yields the sigils recorded in Liber Scientiae. So the Parts, the Watchtower angels and the Tablet of Union names are all readings of one underlying grid, taken in different ways.
That structural economy is worth stating plainly, because it is easy to encounter the Aethyrs, the Watchtowers and the Parts as three separate systems. In Dee they are three views of a single object.