Routine messages (recognizable in military messages by their first letter being "R") are for ordinary traffic, such as peacetime operations or periodic reports, and should be delivered within 24 hours.
Priority messages ("P"), formerly known as "urgent," are for operations in progress, such as updates on troop movements; they should be delivered within one to six hours, and should only interrupt a routine transmission in progress if that transmission is very long.
Immediate ("O," for its old name "operational immediate") messages are for very urgent matters affecting security, such as requests for additional support or reports of unusual major movements of foreign military forces during peacetime; they can interrupt other messages and are handled in 30 minutes to an hour.
And flash messages ("Z," to indicate that they were to be sent through entirely different means – the wire rather than mail) are "reserved for initial enemy contact messages or operational combat messages of extreme urgency… [such as] messages recalling or diverting friendly aircraft about to bomb targets unexpectedly occupied by friendly forces." Rather than having a time limit, these should be delivered "as fast as humanly possible."
The first was that the very successful AUTOVON phone system only used five levels. AUTOVON was one of the first systems to use touch-tone dialing. If you are old enough to remember land lines, you may remember the sound of the buttons: two-note chords, where the low note represents the row and the high note represents the column on the touch pad. AUTOVON added a fourth column for precedence keys, which could be dialed before the phone number to get a priority line. This fourth column thus had room for four keys—the four priorities above "routine."
If priority level four is already for "Oh my God, we're being shot at!" and priority level five is for starting a global thermonuclear war, it's hard to imagine what priority level six might be for
The sixth level was thus named after the two synonyms it most often got in various generations of AUTODIN documentation: "Critical" and "Emergency Command Precedence." Assuming that the only people who would care were people who already knew this documentation inside and out, it was simply given the cryptic label "CRITIC/ECP."The top two levels came not from the military, but from the lessons learned from civilian telephone networks. When a network is overloaded, the most important message to get through isn't any user's message at all; it's a control message telling the network how to manage that load. These network control messages were split into two levels partially because there were still two slots to fill, and partially because of the nature of the new protocol.
Next time you're streaming an old X-Files episode, remember that you're using a mechanism designed to ensure nuclear war could be reliably fought