The msg_type argument in the node.subscribe method is unclear. (Python gz.transport14)

[samh133]

I am attempting to use the node.subscribe method to subscribe to a topic. It appears to require three arguments, msg_type, topic and callback. I have not been able to find in what way the msg_type variable should be passed. There does not appear to be any reference to msg_type in the documentation https://gazebosim.org/api/transport/14/messages.html.
What values of msg_type can be passed?