There are situations when the real submitting user differs from the user what arrives to YARN. For example in case of a Hive application when Hive impersonation is turned off, the hive queries will run as Hive user and the mapping is done based on this username. Unfortunately in this case YARN doesn't have any information about the real user and there are cases when the customer may want to map these applications to the real submitting user's queue instead of the Hive queue.
For these cases, if they would pass the username in the application tag we may read it and use it during the queue mapping, if that user has rights to run on the real user's queue.
UPDATE REQUIRED: Hive jobs are using "userid=" instead of "u=" for the application tags.