When you talk about numbers stations and military transmissions like EAMs, or any traffic on the HF-GCS network, much of the information is not actually known. That does not mean people do not have a good idea how / why things happen, however most of the time there is no actual documented proof you can point to. So be careful with any information you receive, some of it is quite good but some of it is junk, and proving either is pretty hard.
First thing you have to remember is that EAMs and Skykings are not numbers stations. Since they are not, they probably will not have similar habits to numbers stations. And indeed they do not. And then you must remember that such messages, the format of them, and any confirmation of what / how they are used, is classified information, so people who actually know are probably not going to say much. I can say that when I was active duty I occasionally sent and received messages very similar in format to EAMs (but they were not EAMs), and that is pretty much all I am going to say on the matter.
A numbers station often has fixed group sizes, for example maybe 3, 4, 5, or 6 characters per group, depending on the station. And so this results in a character count of some multiple of this group size. And also note that most numbers stations typically do not mix numbers and letters, they are mostly all only numbers, but when they use letters they are typically all letters. This may all be a result of the type of encoding that is done to this traffic. Numbers stations also typically follow specific schedules, HF-GCS traffic typically does not.
But coded HF-GCS traffic is not a numbers station, and an EAM might have any number of characters, and so it will not have specific groups. And the HF-GCS messages mix numbers and letters, however some numbers and letters are never seen. Why is this? Speculation, but I believe it is because the content is different and this traffic is encoded in an entirely different way from numbers stations. I think the smallest number I have ever heard is 8 or 10, and the largest number on the order of 250+ characters, and nothing that falls into any specific group size multiples.
Recently, 24 character messages with a 6 character ID has been the most common format.
An example of one sent 0639 UTC yesterday (7 June, 2018):
ALL STATIONS, ALL STATIONS, THIS IS ROADSIGN, ROADSIGN, BREAK
SLTSGA, STANDBY, SLTSGA, STANDBY, SLTSGA, STANDBY
MESSAGE FOLLOWS
SLTSGAEBJYYK2ELUFQNIQUUEIYGUKA
I SAY AGAIN
SLTSGAEBJYYK2ELUFQNIQUUEIYGUKA
THIS IS ROADSIGN, OUT
If you break this down you see the message was from ROADSIGN (or maybe ROAD SIGN) and a callup of SLTSGA. Maybe SLTSGA identified the message as a specific kind of message, or maybe it identified a specific recipient or group of recipients.
The message itself has 30 characters, but the first 6 match the callup. So possibly the message is 24 characters, with the callup included in the presentation, making 30. But I have heard this same format used with many more, and sometimes less, characters total.
A slightly different format also shows up, for example, this message on 5 June, 2018, at 1946 UTC:
ALL STATIONS, ALL STATIONS, THIS IS DANDY DAN, DANDY DAN, BREAK
FOR 825, FOR 825
KMY2OA, STANDBY, KMY2OA, STANDBY, KMY2OA, STANDBY
MESSAGE FOLLOWS
KMY2OA5JK33WU7TCAOA443W275AOVC
I SAY AGAIN, FOR 825, FOR 825
KMY2OA5JK33WU7TCAOA443W275AOVC
THIS IS DANDY DAN, OUT
Again we see 30 characters total, and a 6 character callup, making for possibly a 24 character message plus callup / message type / recipient ID. Obviosuly though, they have a specific recipient, or group of recipients, in mind, the "FOR 825" is pretty clear.
Or maybe both cases were 30 character messages, and they just break out the first 6 as a callup out of habit, but I personally don't think that is the case.
As for what is a SKYKING, that is a point of contention. We know, from official documents and from observing habits, that a SKYKING is a higher priority message than either normal traffic or an EAM. Some documentation ties SKYKINGs to SIOP (Single Integrated Operational Plan) which is part of the control and use of nuclear weapons. But we also have good indication that SKYKINGs can be tied to other highly important platforms and uses, such as unarmed reconnaissance aircraft.
We also know that SKYKINGs are heard less frequently than other types of messages. You average day on the HF-GCS network will probably have several, if not many, EAMs. Some times you may have half a dozen, or more, in a single hour. But you can go days without a SKYKING, or you may have a dozen SKYKINGs in a single day.
And then it is a safe bet that some EAMs and SKYKINGs are fake traffic, filler messages transmitted when there is no need for them, just to prevent anyone from using traffic analysis as an indicator of force activity.
So I think it safe to say, SKYKINGs are, when real, more important messages than other HF-GCS traffic, and that is really all we know about them.
Some people think if they hear a SKYKING that means something important is happening or about to happen, but to date no one has been able to tie either EAMs or SKYKINGs to world events. For sure there have been 7+ SKYKINGs in the last 7 days and we are all still here.
T!