Final answer:
The speed of service refers to the urgency of message handling, and a priority message in radio communications is marked with a prosign indicating it is of high importance but less urgent than emergency messages.
Step-by-step explanation:
The speed of service in the context of communications, especially in radio and telegraphy operations, refers to the priority or urgency with which a message needs to be handled. In terms of Priority messages, these are often marked with a specific prosign or code that indicates that they have precedence over routine traffic but are less urgent than emergency or immediate messages. For instance, in amateur radio, the prosign for a priority message could be 'P' followed by the message itself. It signifies that the message should be transmitted as soon as possible but does not require immediate interruption of other services.