what is the difference between CLOSED_STATES and CLOSED_STATES_GROUPS in RTC Expressions ?

Hello All,

We have a usecase wherein, we need to sync the severity of the defect workitem to problem workitem,
We have the following code . where we  get both closed and invalid workitems

  IQueryableAttribute stateAttribute = QueryableAttributes.getFactory( IWorkItem.ITEM_TYPE).findAttribute(projectArea,IWorkItem.STATE_PROPERTY, auditableCommon, monitor);
  Expression closedStates = new VariableAttributeExpression(stateAttribute,
      AttributeOperation.EQUALS, new StatusVariable(IWorkflowInfo.CLOSED_STATES));

what is the difference between CLOSED_STATES and CLOSED_STATES_GROUPS.

Request you to please assist on this.

Thanks.