XenApp servers are organised into zones which would normally represent geographical locations within your organization. So you may have a data centre in London with XenApp servers and a data center in Manchester also with XenApp servers. In which case, you would create two zones. Each zone would have its own elected data collector. The data collector, or ZDC, is responsible for maintaining the dynamic load information for the farm. Each XenApp server within the zone reports using IMA and port 2512 to the ZDC of its zone its current load information. This way new users connecting will connect to the least busy servers and users with existing session connect back to their same session. It would make sense to have at least two, probably three, session controllers in each zone. Remember that with XenApp 6.5 only servers that hold the session controller role can participate in the ZDC elections.
Currently our farm has one zone and two servers; XA2 though is a session host only XenApp server. We will remove XA2 from the farm and add it back in with the session controller role. We will see how we can control the election process using the election preference settings for each zone and monitor election process with performance monitor counters. Stopping the IMA service on XA1 will cause XA2 to win the election, starting the IMA service on XA1 again will see the election fall in favor of XA1.