Intensive Data: Data independent of the spatial units (e.g., population density, percentages); data that has been normalized in some fashion.

Extensive Data: Data dependent on the spatial units (e.g., population totals)

You can add spatial units together that have extensive data, but you must use intensive data if you wish to work with subpolygons (division).

This is further discussion of intensive and extensive variables in your textbook: Chapter 4: Technical Box 4.3.