Suppose you have two entities: Person and Rental, and 2 associations between them: Person_IsDriverOf_Rental and Person_IsPayerFor_Rental. The message definition automatically uses the entity name as exposed name, postfixed with a number if multiple associations to the same entity exist. In this case, starting from Rental, the exposed names become Person and Person_2.
In case there is only one association between the 2 entities, this default behavior is just fine. But for complexer models, such as the one before (not so complex!), renaming the exposed names so that the output (JSON) is often desired. However, this is quiet a hard task: the field to edit the name doesn't work properly, and sometimes the renaming is being undone – not sure when or why exactly, seems to happen randomly.
I would suggest using the name of the association as external name instead, as it provides more information/clarity to the reader – even in case the reader is a computer