As per Gabe Aul, this update makes Microsoft target your device in correct ring. One must install this update, in case one has changed Fast to Slow or Slow to Fast rings at some point in time.
@embioeng It’s a configuration pkg that allows us to target your device in the right ring. Get if you’ve switched Fast/Slow at some point
— Gabriel Aul (@GabeAul) September 22, 2015
A new “Insider Fast Configuration update” is rolling out to Windows Insiders on Build 10536.1004. You will receive this update if you are enrolled to “Insider Fast” or commonly known as Fast Ring in your Windows Insider app. So open your Windows Insider app and enroll to Insider Fast, if you are not receiving this update.
Coming to what this update brings in terms of changes, there is no changelog and no visible changes but Microsoft may have fixed some bugs or modified the configuration of its Fast Ring for Mobile devices. Hopefully it means Microsoft is planning to push some new build soon as we are approaching the official launch event on October 6.