Preference Variable in Oracle BPEL 11g

How to setup Preference variable in Oracle BPEL 11g ?
Edit composite.xml to have the preference variable property under bpel component section property name as bpel.preference.
ex: true

Use this property in the BPEL with xpath function ora:getPreference() to access value

During runtime the properties can be modified in em on SystemMbean , soa_infra.. to the Composite, and update the variables.