work items of substitute user

Hi Friends,

i have put substitution to particular user in his absence using RMPS_SET_SUBSTITUTE transaction.

Now how can we differentiate the work items which belongs to actual user and substitute user.

how can we generate the report which mentioned actual user and substitute user for the work items.

need to provide the report to the client because some times when we remove the substitution the work items are not going back to the inbox.

kindly let me know the reason...and solution for the above report.

thanks.

ramesh.