When you configure a georedundant MPE or MRA cluster, you initially set the High Availability site preference to Normal to designate that the primary site is preferred. This determines which site contains the active server and initially processes traffic.
After the servers are defined, you can reverse this preference, which designates that the secondary site is preferred. Reversing site preference makes the spare server take over as the active server. The former active and standby servers become the standby and spare servers (which server assumes which role is not determined).
Reversing site preference is useful in situations where you need to troubleshoot, service, upgrade, or replace the active server.