The quota (1 or more) to allot for a particular source-destination combination is selected by the system only, based on the configuration for that train. The booking staff does not have any control on it.
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The only way to avoid PQWL is try booking ticket till such a station for which tickets are alloted from GN quota. It may happen that berths may be available in GN quota, and you need not even go for a WL.