Segmentation, as a television targeting strategy, is not complicated. A national advertiser can reach the largest number of viewers for the lowest CPM (cost per thousand impressions) on broadcast and cable networks. One advertising message is distributed within a program to all television households that receive that network.
National advertisers may also utilize "spot" television. A spot television strategy takes advantage of Nielsen Media Research's DMA (Designated Market Area) measurement. DMAs typically coincide with metropolitan areas or, more accurately, to the geography that could be covered by a television station's antennae signal. There are 210 DMAs in the United States. The currency for spot television is CPP (cost per rating point). If CPP and CPM were equivelized it is less efficient (more expensive per viewer) to buy spot television than network television. If 50% of a luxury car brand's sales are in 8 DMAs it would then become effective to use spot television in those DMAs.
While DMAs are still the standard Nielsen geographical universe measurement for spot television they are completely irrelevant for geographical television segmentation because less than 20% of all households receive their television signal via rabbit ears. The rest have wired cable, satellite or telephonic fiber optics. Cable systems can segment DMAs by "head end" (usually ranging from 2-3 head ends in a DMA to 60-70) or provide "interconnects" which group all head ends in a DMA together in order to model the antiquated TV-station DMA geography and best compete for spot television advertising dollars.
In a world where there are over 2400 cable head ends, where data analytics can measure the targeting leverage of sub-DMA segmentation, and media vendors can execute segmented media schedules DMAs lose their relevance. If only an industry-standard marketplace existed.