Message precedence is an indicator attached to a message indicating its level of urgency.
CCEB military precedence
The Combined Communications Electronics Board (CCEB), a five-nation joint military communications-electronics organization (consisting of Australia, Canada, New Zealand, the United Kingdom, and the United States), uses the following message precedence designators, in descending order of importance:
This precedence is reserved for initial enemy contact messages or operational combat messages of extreme urgency. Brevity is mandatory. FLASH messages are to be handled as fast as humanly possible, ahead of all other messages, with in-station handling time not to exceed 10 minutes. Messages of lower precedence are interrupted on all circuits involved until the handling of FLASH messages is completed.
This precedence is reserved for messages relating to situations gravely affecting the security of the nation. It requires immediate delivery. Examples include reports of widespread civil disturbance, reports or warning of grave natural disaster, and requests for or directions concerning search and rescue operations. IMMEDIATE messages are processed, transmitted, and delivered in the order received and ahead of all messages of lower precedence. They are to be handled as quickly as possible, with in-station handling time not to exceed 30 minutes. Messages of lower precedence should be interrupted on all circuits involved until the handling of the IMMEDIATE message is completed. The use of the letter "O" comes from the original name for this level, "operational immediate".
This precedence is reserved for traffic requiring expeditious action by the addressee or for conducting operations in progress when ROUTINE precedence will not suffice. PRIORITY precedence messages are processed, transmitted, and delivered in the order received and ahead of all messages of ROUTINE precedence. Examples include requests for supplies or equipment during the conduct of an operation, time-critical items requiring quick response, and situation reports. They are to be handled as quickly as possible, with in-station handling time not to exceed 3 hours.
This precedence is used for all types of message traffic justifying transmission by rapid means, but not of sufficient urgency to require higher precedence. ROUTINE precedence messages are delivered in the order received and after all messages of higher precedence. Examples include any message that requires the documentation of its transmission or delivery; messages concerning normal operations, programs, or projects; and periodic or consolidated reports. They should be handled as soon as traffic flow allows, but no later than the beginning of the next duty day.
Flash Override (Y)
The National Command Authority (usually the President of the United States) has access to a FLASH OVERRIDE (FO) capability. FO is not a precedence, but instead represents the authority and means to override all other traffic, including FLASH precedence messages. In written message traffic, the proword 'Y' is used to indicate a message having the authority to override all other traffic and is usually assigned to Emergency Action Messages (EAM).
Other precedence systems
The amateur radio community uses EMERGENCY, PRIORITY, WELFARE and ROUTINE precedence levels.