What is the difference between "set community" and "add community" in juniper?

Question

I am trying to understand setting up policies on a juniper router. What is the difference between "add" and "set" community?

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

Answer 1

set replaces the current communities on a route with the specified community, while add adds the specified community to any communities already present on that route.