Thanks once more, Noght & Fidelis, for taking the time to answer.
-=-=-
About special rules:
Special Rules for a unit are in the AB entry. @ Noght: see Fidelis' quote, it says "Most" special rules are such, not "all".
@ Fidelis: what you quoted is a list (not necessarily exhaustive) of how to get a special rule. Steadfast can be granted by terrain indeed, +1 credit to your honesty.

To define a special rule to be an AB entry, or to be derived from a list, that's another of your interpretations.
It is not GW's definition (see p.66 if you're in doubt).
My definition of a special rule is just derived from p.66, top "...we have special rules - uncommun rules to govern uncommun circumstances". At least, this is a definition that GW has printed.
So of course, stubborn is a special rule, steadfast is a special rule too.
Anyway, even if SF would not be a special rule, so what?
How does it make stubborn so special that, despite stubborn being exactly steadfast in its effects, you urge me to ignore stubborn troops, which make clearly an exception to your theory?
According to the Light Troops rule on p. 77, skirmishers always count as having zero ranks, and therefore cannot be steadfast.
And yet, they are always Stubborn in woods. Why? Because the rule that governs the terrain feature makes this clear (see status above).
Note also that units that normally could be steadfast, cannot be steadfast here – unless they are Stubborn.
BRB p.76: "Stubborn units are always steadfast, whether or not they have more ranks than their enemy."
If you don't even accept to see the contradiction in your argument with GW printed material, I cannot help you.
You should ignore Stubborn rules, it seems confusing to you. That's 51.
(make it 52 as I quoted it)

-=-=-
About the definition of SF,
there is one common ground on which everyone should be able to agree to be THE definition of SF:
That is to take p.54,
everything starting from
"STEADFAST
If a defeated ..."
till the bottom of the page,
"... Leadership value."
Could you agree on that?Moving from that,
retaining one sentence as THE definition, discarding all other sentences as fluff, subcases, particular cases or whatsoever is already making an interpretation (not telling yet whether it is valid or not).
Could you agree on that?-=-=-
Anyway, besides the definition of SF, let's not forget that we're examining what happens in the following case:
Q: A detachment has to take a break test. The parent unit is at 3" but not in the same combat. Is there a possibility that the parent unit be steadfast?
Let's take the case through the whole subchapter steadfast.
"If a defeated unit..."
If <2 conditions> then <consequence>.
<2 conditions> = more ranks + defeated. The word defeated is there indeed.
When the two conditions are met, the sentence tells what to do.
Whenever either one of the two conditions is not met, the sentence tells you... nothing.
One side takes a portion of a rules that tells nothing as a proof that the whole sub-chapter cannot be used to answer our case.
The other side takes it as a proof that for the case we're discussing about, we need to pursue our investigations.
"When at war..." We probably can all agree that this paragraph is a little vague, so there is hardly any way to use it and tell whether a unit can be SF or not.
If you insist, we can review it carefully to check if it may apply or not to our case.
"Simply put..." Here is a sentence and a paragraph which describes precisely a situation. One side refuses to use this paragraph. The other side is eager to make good use of this paragraph and provide the answer, pending no contradiction is found anywhere else in the definition of SF.
"Steadfast units can always take brake test..." Here, the unit can take a break test, but doesn't have to. It brings no knowledge to understand our case.
Here is pinpointed the major difficulty in Noght's and Fidelis' thesis:
How could they use a sentence self-limiting to defeated situations, to tell what happens outside of a defeated situation?
How could they ignore the rest of the definition of SF, which very clearly deals with the very situation we are discussing about?
-=-=-
@ Fidelis,
Sorry, we were typing at the same time,
I did not have a chance to see your whole message.
EDIT:
- your tautological conclusion comes from missing (or ignoring) the word "either" (see below where it appears).
- If you wish to mention "having the steadfast status" or "benefitting from SF rule" instead of "being SF", why not?
However, telling that a unit would "have SF rule" seems to imply that the rule is written in its AB entry. I would not recomment such wording.
A unit can "have stubborn rule" by AB entry, or "benefit from stubborn rule" by circumstances (crown, skirmisher in woods, night goblin in mushroom wood, detachment within 3" of greatswords parent...); a unit can "benefit from SF rule". As we know that SF is a status (see top p.55), "benefitting from SF rule" is strictly equivalent to "having SF status".
If you really wish so, my definition of SF could become:
Definition of
having SF status: A unit
has the steadfast status if either one of the following conditions is met:
1. more ranks 2. in a building 3. stubborn 4. detachment within 3" of steadfast parent unit.
How to use SF status: When losing combat, a unit
having SF status passes break test on Ld, ignoring combat modifiers.