Olá,
a descrição desses campos está disponível no arquivo moodle/lib/db/install.xml
<FIELD NAME="component" TYPE="char" LENGTH="100" NOTNULL="true" SEQUENCE="false" COMMENT="plugin responsible responsible for role assignment, empty when manually assigned"/>
<FIELD NAME="itemid" TYPE="int" LENGTH="10" NOTNULL="true" DEFAULT="0" SEQUENCE="false" COMMENT="Id of enrolment/auth instance responsible for this role assignment"/>