As defined by OSHPD "rural" is a designation of any MSSA which has a population density of less than 250 persons per square mile, and which has no census defined place within the area with a population in excess of 50,000. Only the population that is located within the MSSA is counted in determining the population of the census defined place.
Source and for further information: California Healthcare Workforce Catalog http://gis.ca.gov/catalog/BrowseRecord.epl?id=23784
Territory, population and housing units not classified as urban. "Rural" classification cuts across other hierarchies and can be in metropolitan or non-metropolitan areas.
Source and for further information: U.S. Census Bureau http://factfinder.census.gov/home/saff/main.html?_lang=en
Urban Commuting Codes
These codes classify census tracts using measures of urbanization, population density, and daily commuting. The classification contains 10 primary and 30 secondary codes.
Source and for further information: USDA Economic Research Service http://www.ers.usda.gov/briefing/Rurality/RuralUrbanCommutingAreas/