The Message and the Medium
As ThePinkUn points out the Medium is NOT the Message
I have attempted to avoid getting too geek, but hope the following is useful.
IATA/ATA Have a entire series of structured message Formats for all most all types of Operational, Planning, Pax/Cargo and Engineering Messages. Many of these have evolved from their original very simple teletype formats to reasonably sophisticated message structures, Whilst these formats are technically recommendations, in many cases they have become de-facto standards.
They don't specify the medium over which these messages are sent. But many of these will be required to be high priority so marking the message as such for transport medium handling is important.
ACARS/AIRCOM Messages whilst following a structure, tend to be more Airline bespoke than IATA/ATA formats, the exception being the IATA MVA format set for ACARS/AIRCOM Transmission
Free Text Messages
Again the Transport Medium used is not important, but where a high priority is required the message to be marked as such.
IATA coordinate the provision of base addresses for TypeB and Similar Networks (SITA/ARINC) . In simple terms the full address is made up of Location/Office/Company:
Location: e.g. LHR, Office: e,g, KK, Company: e,g BA
So the example Address is LHRKKBA.
Whilst this address type is for TYPE B Network Use, it is useful in other circumstances as if can be used by company computer systems to look-up how to actually route a message (Internally over the LAN, Over SITA/ARINC, as an email, package up as an AFTN, or other Type of Network Format, to another computer via TypeA, or other WAN/LAN service, or if outbound even to a fax - Which I have seen done).
For SITA/ARINC and Similar Networks Priority is both a indication of how the message is handled and how much it is going to cost. Once again retaining the priority code can be useful for other transport mediums as it may actually be used by the receiving computers for handling, even though the transport medium itself makes no use of it
Obviously the highest priority is where Life is endangered (QS and alternatives). The one we all would rather never have to use, or receive. With SITA this is given ultra special handling. Other than specialist Aeronautical and Military Networks, this type of priority doesn't exist, do you can only use the highest priority (it any) a network offers!
Even if at some point a Message is being carried over SITA, or similar Network, it can enter the network via AFTN, IP, Telex, or any network that has the ability to transfer message onto SITA, or similar.
If the message is being sent as email, or a Network outside SITA, or Similar then the protocol(s) being used will define how the message is packaged,
SITA and ARINC TypeB - The one advantage of this is simply Guaranteed Delivery. Uses store and forward techniques to guard against message loss..
Mix and Matching
These days a single message maybe routed internally by one means and externally as a mixtures of methods: e.g. IP as direct connection between computers, or as email, TypeB, or in some parts of the world Telex, or other network.
For the sender and recipient the only requirement is that it transmitted and received accurately and in timely manner.