Final answer:
The speed of service for a Flash message is the highest priority and requires delivery within 10 minutes. Prosigns for Flash vary, but 'Z' is commonly used to indicate its status.
Step-by-step explanation:
The speed of service for a Flash message is the highest priority in military communications, typically reserved for messages that require immediate action or that pertain to commanding officers for operational decisions. The prosign or procedural signal for a Flash message varies depending on the specific military communication protocols in use, but in some systems, it can be designated by the prosign 'Z', which precedes the message to indicate its priority status. The expectation is that Flash messages are to be delivered within 10 minutes of their transmission.