The IEEE "Technical Activities Board" (TAB) defines the "IEEE Societies" and the "IEEE Divisions" in its "Technical Activities Board Operations Manual"
provides a forum and community for the exchange of technical information among members in the respective discipline and industries of electrical and electronics engineering and related arts and sciences. Societies may represent either an identifiable segment of the electrical and electronics industries, or a discipline having applicability in different industries and functions. A Society's scope or mission shall be approved by TAB.
(page 89, section 4.2 of the TAB).
As for Divisions...
There shall be ten Divisions. A Division is typically composed of one or more Societies, and may contain one or more Technical Councils. The Technical Activities Board shall determine the specific composition of each Division. The composition of the Divisions shall be specified in the TAB Operations Manual. If a Society warrants representation by more than one Division, the voting members of that Society shall be voting members of all those Divisions
(page 86, chapter 4.0 of the TAB).
The above is followed by the listing of the ten Divisions.
Most Divisions have several associated Societies. The IEEE Computer Society is unique in that it is associated to both "Division V" and "Division VIII" and this exclusively (i.e. there is no other Society associated to either Division V or Division VIII).
Why is that? Is it only because of the size of the IEEE Computer Society?
As an aside, note that elections of the Division Delegates (who are also Directors on the IEEE Board) take place each year but alternate between odd and even-numbered Divisions. Thus, this year the Delegates for Division V are being elected, whereas last year the Delegates for Division VIII were elected.