Using Resource Manager question

Hi I have created a resource manager group.

This is one of the plan directives I have in place and I have it in place because I understand it is mandatory.

dbms_resource_manager.create_plan_directive(

      plan=> 'USF_ORACLE_COM_PLAN',

      group_or_subplan => 'OTHER_GROUPS',

      comment => 'The mandatory group',

      mgmt_p1 =>50);

I am assigning users to other plan directives.   My question is:  For all users that are not explicitly assigned to other directives, do they automatically get assigned implicitly to the 'Other Groups' plan directive?   Is this an all or nothing deal?   As I'm understanding is that once you enable Resource manager that all users in the database get assigned to the resource group and that is why an 'other groups' has to get defined.   Am I understanding this correctly?  Please advise.