As this is nowhere documented - I really would appreciate getting an info about this too.
I talked to support about this, it should be true by default. Setting it to false should prevent appservers from sharing work item threads, mainly for troubleshooting purposes. so the default is for multiple appservers to share the WITs across multiple servers/instances.
You may want to open a ticket w/ support on this as well if you have more questions about how to use it (and to request it is documented)...
thank you, Bill. The issue-id for the RFE is: ISS03535088