From what I've Ceen in the Dave Canterbury 5C/10C survival lists the most important C, The Zeroth C, Communication, is missing from the list.

I suspect it is missing due to the marketing aspect.  Getting found, fast, doesn't really bring the other items into play.

But when you research systems, such as the pilot's survival vest, you immediately see that signaling, ie: getting found, represents half, or more, of the components in the system.

So I submit that The Zeroth C, Communication, be first and foremost in your system and the most underrated item in Communication is the:

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SIGNAL MIRROR:
I would recommend either size of the SI Howard tempered glass mirror.  But any other mirror, even the plastic ones, when wrapped in something as simple as a couple of layers of cling wrap to protect the reflective surface, will last forever inside a vest/pack/container.

FLAT WHISTLE:  
This is another item that will never expire and without a pea in the whistle it will operate fine in any temperature.  They are so inexpensive and compact that adding several to your system is a simple matter.  Keep one on a backpack strap, another in your pocket survival kit, etc.

Beyond these two Communication items can also mean using fire to signal (green material on a fire makes white smoke) and so on.

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Then we have electronic means of Communication, the most obvious being a:

RADIO.  
I once purchased a Yaesu FTA-230 ht (handheld transciever) to keep in my flight bag and I consider it to be a near-perfect survival radio.  Water resistant, great battery life, with an optional AA backpack in which I've put Energizer lithiums, and has a dedicated 121.5 button.

I like this much more than a marine radio, which only operates on the marine bands.

VHF radios, such as these aviation and marine radios, are line of sight.  On flat ground/open water your line of sight is about 8 miles, less with obstacles in between; buildings, mountains, etc.

But line of sight from the ground to an airplane overhead is effectively unlimited.  
(Line of Sight refers to the ANTENNAS.  This is why a sportfish yacht, with the VHF antenna way up on the tower, can have an LOS of +20 miles.)

PLB - Personal Locator Beacon:
These require registration with NOAA, which is free, and free renewal every two years.  Registration and renewal can be done online.  The SPOT locator is a paid subscription system, which, while better than nothing, I don't recommend it.

The model I have is the ACR ResQLink+ PLB (the Plus model floats).  It is approximately the size of a 40mm grenade , but slightly larger unfortunately.  It doesn't fit in a 40mm grenade pouch .

This PLB transmits on 406Mhz which is a satellite based system so line of sight is upward.

Newer models of 406 PLBs can also have a text function, which requires a separate, paid, service.  I do not know if these services and the SPOT service  are related.

STROBES:
Your Candlelight from the 5C/10C list may have this option in its flashlight functionality, which is nice.  I personally like the flat strobes from ACR, like the Firefly, and my favorite ACR strobe is the one they make for the military, the MS-2000.  All these flat strobes use AA batteries and will function fine with the slightly higher voltage lithium AA cells, which is great as these tend to last a decade or more in a strobe or flashlight without leaking (my personal experience).

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Then we have chemical means of Communication:

SMOKE and FLARES:

There is a dizzying array of smoke and flare options.

Smoke is good for making yourself 'larger', ie: easier to spot.  It allows someone to follow the smoke to your exact location, ie: the source of the smoke.  (another option it allows is to help a pilot to determine wind direction and speed)

The only flare option I'll illustrate here is the pen gun style.  While the military has a great one that gets up high, it is near impossible to buy.  
Fortunately Orion makes a decent, near-equivalent, pen gun option, same basic design, called the Orion Pocket Rocket.  It is most easily found at Bass Pro (this way you can save on hazmat lol).  FYI: Take a real close look at the expiration date on the 'rockets' so you can grab the pack that has the furthest date.

The aerial flares aren't rockets though.  I tested one.  First; the primer is super hard .  It took four, full, strikes to get the flare to launch.  It sounded like a .38Spc going off and it appears that the primer provides ALL the 'propulsion' to launch the flare into the air.  I did look for the flare afterward but couldn't find it.  Before and After photo down below.

The system comes in a tube with three aerial flares.  I bought several so I can have more flares and then sewed up a piece of stretch webbing and used my foodsaver sealer heat function to 'stamp' out a sheath of sorts

I clicked this thread because I had no clue what the hell you were talking about. Your title makes no sense.

with that said, "communication" aka signaling has always gone hand and in hand with survival that most ppl understand, my thoughts anyways.

I keep a mirror for multi reasons - first aid (got something in your eye), Fire starting (reflection of sun) and signaling (either thru fire or flashing the mirror itself.)

In fact, I got booted from frugal squirrel many years ago because one of the mods didnt like my answer why you SHOULD keep a mirror and it could be just as important as everything else in the kit.

Ok, I added 'survival' to the title.

Like adding 'tactical' to anything, the title is now complete

Im still trying to figure out what "The Zeroth C......" means...

There's this new invention, the internet ....

ze·roth
'zi(?)roTH,'ze?roTH
adjective
immediately preceding what is regarded as first in a series.

Well, it's a slick way to make it the "11C's" instead of just 10, by counting 0-10

It actually makes sense.  I would more liken that "C" to be part of my "edC"...what you "Every Day Carry", but I can understand the absence of communications as well.  It's not hard to EDC your phone, whistle, small LED light, and even a mini-signal mirror.

At the end of the day, it's still a balance of basic essentials built around your own skillset, experiences, and requirements...

ROCK6

I carry pen flare and cartridges mainly because they do not depend on sun light.rain, fog and heavy canopy are consistent issues to think about in the NW.

It's definitely a necessary consideration for dense tree cover or narrow, deep ravines.  I packed one when we did some hiking and ravine exploring in Utah.

ROCK6

Nobody seems interested in the other thread about pyrotechnics so I'll add the video here