> The U.S. Office of Management and Budget defines a set of core based statistical areas (CBSAs) throughout the country. CBSAs are delineated on the basis of a central urban area or urban cluster – in other words: a contiguous area of relatively high population density. CBSAs are composed of counties and county equivalents.[3] The counties containing the core urban area are known as the central counties of the CBSA. A central county is a county in which 50% of its population lives in urban areas of at least 10,000 in population, or where a population of 5,000 are located in a single urban area of at least 10,000 in population where that urban area is split between more than one county.[4] Additional surrounding counties, known as outlying counties, can be included in the CBSA if these counties have strong social and economic ties to the central counties as measured by commuting and employment. Outlying counties are included in the CBSA if 25% of the workers living in the county work in the central county or counties or, conversely, 25% of the employment in the county is held by workers who live in the central county or counties. All counties in a CBSA must be contiguous, and a county can only be included within one CBSA.[4] In New England, towns have precedence over counties, so statistically similar areas are defined in terms of town-based units known as New England city and town areas (NECTAs).
> Adjacent CBSAs are merged into a single CBSA when the central county or counties of one CBSA qualify as an outlying county or counties to the other CBSAs.[4] One or more CBSAs may be grouped together or combined to form a larger statistical entity known as a combined statistical area (CSA) when the employment interchange measure (EIM) reaches 15% or more.
https://en.wikipedia.org/wiki/Metropolitan_statistical_area
San Jose and SF are combined in a combined statistical area, but the linkages are not strong enough to consider them a single metropolitan statistical area.