Yeast generations

i've been reading 'brew like a monk' recently and i'm confused about yeast generations. most of the breweries in there mention they don't use yeast past a certain generation because of mutations, off flavors, etc.

that i get, but how is a "new" generation made? it seems like if you use a yeast after an initial run, all subsequent uses are the "next" generation. i also read about "top cropping," is this the method to secure a new "first" generation yeast?

thanks.

They do something like this I suspect - Yeast bank - Home Brewing Wiki

I believe most people would call a yeast grown directly to starter from a lab slant or tube to be first generation. Rather than yeast re-pitched from a previous brew.