http://i3xi5qxvbrngh3g6o7czwjfxwjzigook7zxzjmgwg5b7xnjcn5hzciad.onion/proposals/128-bridge-families.html
If a bridge user knows about only one private bridge, and that bridge
shuts off for the night or gets a new dynamic IP address, the bridge
user is out of luck and needs to re-bootstrap manually or wait and
hope it comes back. On the other hand, if the bridge user knows about
a family of bridges, then as long as one of those bridges is still
reachable his Tor client can automatically learn about where the
other bridges have gone.