In earlier versions of Solr, there was both a <queryParser> named “dismax” as well as a <requestHandler> named “dismax”, setting the defType parameter to “dismax”. This caused some confusion so it was removed in 3.1 (see https://issues.apache.org/jira/browse/SOLR-2363).
To learn more about DisMax, please see the official Solr Reference Guide: https://cwiki.apache.org/confluence/display/solr/The+DisMax+Query+Parser