What does msg mean in Mirth?

I'm trying to learn my way around some Mirth code. I keep seeing things like this in the transformer for di开发者_运维问答fferent channels:

msg['PRB']['PRB.4']['PRB.4.2'].toString()

I understand that this is parsing a HL7 message and I understand that msg probably means message. But is MSG a system-wide variable? Does every channel have a msg variable? What is the scope of msg? I also keep seeing things like ${message.encodedData} in the Templates. Do certain connector types automatically send the contents of msg?

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The msg variable is the current instance of the inbound message that you will use for routing or mapping. It only exists in the context of the mapping/transformation that you are applying and is not system-wide. The message that will be transmitted is referred to as tmp. So, when you are mapping, you will be going from msg -> tmp.