My understanding of the concept is based on the Swedish Flowers, which apparently survived on farms in 3 villages in southern Sweden. To try to preserve them the Swedes developed/use the concept of a living gene bank, which uses certificates to track the lineage of a bird back to the members of the surviving flocks when the scheme was started. It is described here
https://www.kackel.se/den-levande-genbanken/
I understand the motivation and the desire to preserve local village chickens, wherever they are, especially their genetics, but this approach seems to turn them into a pseudo-pure breed. It excludes all Swedish Flowers that do not have a paper trail back to birds in the villages of Vomb, Tofta or Esarp but otherwise look and live like them, as mine do [an excerpt from the page translated by Google is relevant and fun: "The gene bank hens must be allowed to live and bounce on people's manure piles and this is not so remarkable, this with gene bank. It is actually possible to help preserve a unique little remnant of our cultural heritage by having something as straightforward as a small flock of chickens that basically takes care of themselves"], and it's not obvious to me that a certified paper trail helps preserve them better than wide distribution in a lot more little flocks in a lot more places, where they can potentially evolve to suit those conditions just as well as the originals did in southern Sweden.