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?

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.