The content of data exchanged between entities can be a low-level message containing sender and receiver address, a message number and a simple byte buffer, or it can be on the higher abstraction level of a method invocation on remote objects. While the latter seeks to provide transparency by extending the procedure call paradigm to remote group objects (RPC), the former would typically be used when sending unstructured data (e.g. audio, video) over a channel.
Note that remote method invocation is mostly synchronous while message sending can be synchronous or asynchronous.