Hi Subhojit,
You have to use a context variable in an XPath to retrieve data:
[ActionedBy = $RetrievedUser/Name]
If you want to retrieve data over association you can for example use:
[GlobalTracker.DocumentIdentifier_User = $RetrievedUser]
not sure what you try to achieve.
do you want to SET the association or CONSUME it in a retrieve?
In any case; I would never use a direct association between some data in your process and your user.
Always use a Entity person/customer/employee/.. which contain the person data used in the process and a 1-1 association between Account and your person process entity.
You process data (in your case DocumentIdentifier) reference that to your person process entity instead of user.