Third party messages are sent and received by radio operators on behalf of others (third parties). That means that the third party determines:
Message format (usually some type of form used by that agency)
Message content (often including terms that are unfamiliar to the radio operator)
Message to/from information (typically, an ICS position and location)
Message handling order (how quickly they need it sent)
With third party messages, the radio operator must adapt to and operate within the served agency’s workflow, not the other way around. An effective way to think of the radio operator’s job is to compare it to a fax machine. What should come out the other end of the radio channel and be delivered to the message recipient should be a remarkably close facsimile or replica of the message that the message author created. The radio operator must not interpret or change the message and it must arrive at the other end in the correct format and fully legible.
The key operational difference between handling operator-to-operator messages and third-party messages is the need to send the third party messages exactly, so that the message recipient receives what the message author sent. Decades of experience has shown that the only effective and efficient way to do that is to use a shared set of procedures.