SELECT PROMPT,LAST_UPDATED_BY,LAST_UPDATE_DATE FROM knta_parameter_set_fields WHERE parameter_token like 'Test' -- Give you field Token AND product_scope_code = 'RI' and parameter_set_context_id IN ( SELECT parameter_set_context_id FROM knta_parameter_set_contexts WHERE context_value IN ( SELECT TO_CHAR (request_type_id) FROM kcrt_request_types WHERE request_type_name ='Test' )) -- Give the request type name
On the field definition, you can set Notes History to Yes and every change to the field will generate a Notes entry on the request. You will have a complete record of all changes, not just a record of who changed the field last.
To do this in a rule, you would have a read only field for the id of the person doing the update (validation PPM - User Names - All or something similar) and a read only field for the time of the update. The rule would be set for 'Apply on field change' and the one field would be listed in Dependencies. The Updater and Update Date fields would be listed as Result Fields. The query would be something like:
select '[SYS.USER_ID]','[SYS.USERNAME]',sysdate,sysdate from dual