215ers?

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Whats up with the 215 thing in humboldt county? I did a quick google search, but it basically said medical marijuana is legal. I was looking at rentals in Eureka till the wife and I have our house built, and in almost every ad, 215ers are mentioned.... As in not allowed.... Are they basically saying no pot smokers? Is this a humboldt county only thing? I know humboldt is famous for growing herb (thanks homegrown movie), is 215ers common in that area?