a grouping of 16 units
A set is a grouping of 16 units. Although the set contains four separate quads and the units are all in specific positions according to their natures, the quads are not considered in the set. This is because the concept of the set is to define a group of 16 that all have similar natures. In other words, a set is an umbrella that equally covers each of 16 individual units in a group.