The highest possible precedence for a signal message.
For a point of reference, the next lower precedence was IMMEDIATE, which had a target delivery time of 30 minutes, world-wide. FLASH was higher. There was no target delivery time, it was just “as fast as humanly possible”. Incoming FLASH messages would usually have bell characters, so the teleprinters started alarming. The RX op would shout “FLASH in the Commcen” and you basically dropped everything to get this message out to either the next station, or the recipient, if it was for your station. All kinds of special treatments. It was often UNCLAS, to avoid all the special handling associated with classified.
But it could also be classified, so then you’ve got a FLASH message, GOC XX DIV EYES ONLY (say). That subtends helicopter tasks, if he’s out in the field, for example. No question at all, the content of the message could be imminent nuclear attack. Or his wife’s flowers are going to be late
Real ones were quite rare. I happen to know the bloke that received the “Execute” message for the opening attacks in GW1, and he said even that was only IMMEDIATE.
So sending a routine acknowledgment signal with FLASH precedence was a bit of a no no
ETA: Just thought of something else.. The F.Sigs 266 (Message form) had a signature box. The releasing officer should have been a commissioned officer of at least a certain rank for each precedence from ROUTINE, through PRIORITY, IMMEDIATE and FLASH. IIRC WOs could authorise a ROUTINE, so that orderly officers could send a routine acknowledgement out of hours if required, and a WO was ROO. However, there was a caveat. If circumstances demanded it, any NCO, WO or Offr could release a message of any priority, including FLASH. They had to write “Notwithstanding the provisions of JSP 101 Chapter 16 Annex B” under their signature block, and that was it. Message duly authorised. But you’d catch hell if you couldn’t justify it.
Why can I still remember this shit 35 years after I was taught it?
In my entire 30 year career in the signals, I only ever saw one genuine NODUF FLASH message. Saw quite a few on various exercises, but just the one in real-life.
On another note, here's an irritated puppet ripping the absolute shit out of Frenchy Baguetteface's half-arsed attempt at directing an adaptation of one of the finest Sci-Fi novels of all time, Frank Herbert's DUNC:
Makes me glad I didn't get conned into forking out my cash on the cinema or home video version, but also gave me a further appreciation of David Lynch's peak '80's version into the bargain.