Intrastate region

An intrastate region of Mexico (Spanish: Región Natural or simply Región) is a collection of municipalities in a Mexican state that are grouped together for statistical purposes. For example, in the State of Mexico, the 125 municipalities are grouped into 16 regions, and are numbered by Roman numerals. The reasons for such a grouping include simplicity of administration, as well as geographic relationship (such as being part of a particular valley.)

The intrastate regions are distinct from the Regions of Mexico, which are collections of states rather than municipalities within a state.

...
Wikipedia