  
               | 
              
                Valid message
               | 
              
                This message is completely valid.
               | 
            
            
              
                
   
               | 
              
                Message direction different from the PLM connection
               | 
              
                This message direction is different from the direction of the associated PLM connection.
               | 
            
            
              
                
   
               | 
              
                Message target above message source | 
              
                On a vertical axis, the target position of this message comes before the  source position. | 
            
            
              
  
  | 
              Return message with potentially incorrect direction | 
              (Warning) Only for return messages that go from left to right. |