The sets the region is the overlap of, or the one set it is.
OptionalsizeHow big the region is: the set's size for a single set, the size of the overlap for several. Omitted when the diagram is drawn for its membership alone.
OptionallabelWhat the region is called on the diagram, when it is named.
OptionalitemsThe items written inside the region, when the diagram lists them.
One region of a Venn or Euler diagram: a set on its own, or the overlap of several.
A region names its sets rather than being nested under them because an overlap belongs to every set it joins equally; the sets themselves are derived from the regions, as a flow's nodes are from its flows.
Example