A permanent is a brevet you can register to ride at any time, on your own, or with a group of other registered riders.
You must be a RUSA member AND register for the permanents program for an additional fee.
More details here.
You can search for permanents here.
Log in to RUSA to register for permanents here.
Note that you're now allowed to reverse a route and/or start from an intermediate control (and then proceed through all of the controls to complete the route). For example, for the Cleveland-Amish-Devo route, one may start it in Cleveland, Middlefield, or Akron.