我试图了解在瞻博网络路由器上设置策略。“添加”和“设置”社区有什么区别？

set policy-options policy-statement MY_POLICY term MY_TERM then community ?
  <community_name>     Name to identify a BGP community
  +                    Add BGP communities to the route
  -                    Remove BGP communities from the route
  =                    Set the BGP communities in the route
  add                  Add BGP communities to the route
  delete               Remove BGP communities from the route
  set                  Set the BGP communities in the route