Heterogeneous resistance attempts to quantify people with different exposure and transmission levels than others. Heterogeneous resistance often revolves around two variables; age and social mixing to determine susceptibility.
Heterogeneity typically lowers the herd immunity threshold. First the virus infects people who are more susceptible and spreads quickly. But to keep spreading, the virus has to move on to people who are less susceptible. This makes it harder for the virus to spread, so the epidemic grows more slowly than you might have anticipated based on its initial rate of growth. For example; transmission and immunity are often concentrated among the most active members of a population, who are often younger and less vulnerable.
Here is a link to a paper discussing the 20% threshold:
https://www.medrxiv.org/content/10.1101/2020.04.27.20081893v3.full.pdf+html
For his chart:
Connector - is a person who performs a high number of transactions per day
Processor – is a person who interacts with a connector but few other processors
Sink - is a person who does not leave their residence and has minimal interactions with anyone
You read the chart by connecting the detection type and the role. So a Connector-Symptomatic may need 70% immunity but a Sink-Symptomatic only needs 5% immunity. You then weigh each population group by your guess as a percent of the population (20+45+35=100).
*not an endorsement, just an explanation*
TLDR: The virus distribution in a large population is nonrandom, the respective nonrandom immunity in a population lowers the actual herd immunity threshold for the population not measured in many "herd immunity" models.
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