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org.apache.struts2.interceptor.MultiselectInterceptor is in the defaultStack. It checks each form parameter submitted to the action and if it finds one with a prefix of __multiselect_ it inserts a value for a parameter whose name is derived from the suffix to __multiselect_ if it does not exist. The default value inserted is an empty String array.
This means that a a field that can have multiple selected values(select, checkboxlist, etc) can be accompanied by a hidden input with the same name but a prefix of __multiselect_ so that if the no value is selected on the form the action will still receive a value(empty collection) rather than the default HTML action of not providing a value for unselected elements,