Hi Siva,
Resolution notes is not an OOTB field on any level of the request hierarchy so if you have that as a field,it is something that your organization has added. Based on context, I'm assuming that this needs to go on the item record. From there it is simply a matter of setting up a BR on sysapproval_approver to pick up that the item(s) was approved. From there, you have access to the data, I assume, on the sc_req_item record or the sysapproval_approver record. You just need to do a <variable holding your item>.u_resolution_notes.setvalue(<build the string here or build it prior and use the variable name>); In this case you will need to do an update() to the requested item record after the assignment.
Hope that helps.
:{)
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