Glassfish 3.1 default principal to role mapping

I am working with glassfish and jaas module.

I configured my web.xml in this way.

<security-constraint>
    <web-resource-collection>
        <web-resource-name>ALL Page for admin</web-resource-name>
        <url-pattern>/*</url-pattern>
    </web-resource-collection>
    <auth-constraint>
        <role-name>user</role-name>
    </auth-constraint>
</security-constraint>
<login-config>
    <auth-method>BASIC</auth-method>
    <realm-name>file</realm-name>
</login-config>
<security-role>
    <description>Administrator</description>
    <role-name>user</role-name>
</security-role>

It means all user that want to access my web application need be of the group user.

Then on the glassfish console I need to tick the options in: Configuration -> server-config -> security -> Default Principal To Role Mapping

My question is why I need to tick this Default Principal to Role Mapping ? And how I can change my web.xml to avoid to tick it ?

Thanks a lot

Loic