@ Noght
I'm sorry for you if you cannot understand that instructions are rules, definitions are rules, instructions are not definitions.
There is no specific precedence between instructions and definitions, some define "what is...", some tell "how does...", all are rules to be applied.
Back to topic.
When a parent unit, in combat with more ranks,
is not defeated, how do we know whether it is steadfast or not?
We read what rules say. Proposed rules to be used:
" If a defeated unit..."
"Simply put, a unit is considered to be steadfast..."All of these rules are equally valid (as you now seem to acknowledge, finally). But some of them might not provide any answer.
Can we use a rule about defeated units to tell what a non-defeated unit is?
Can we use a rule about steadfast units to tell what a steadfast unit is? Why do you refuse to use the rule "Simply put..."? You're not cherrypicking, are you? Not you?

@ Fidelis
Let's review, once more, sentence (1) as you propose.
My quote: "replace
“has more ranks than its enemy" by
"is steadfast". "
+
“If a defeated unit has more ranks than its enemy, it takes a Break test without applying the difference in the combat result scores."=
“If a defeated unit is steadfast, it takes a Break test without applying the difference in the combat result scores."When I do it myself, it looks better.

In your sentence (2), you did not do what you said you would, i.e.
"substitute the word steadfast in the Stubborn rule by the given definition",
when your own definition was
"2. Noght, Fidelis & co definition of steadfast = defeated, having more ranks than its enemy, taking a Break test without applying the difference in the combat result scores"The BRB rule about stubborn is:
"Stubborn units are always steadfast, whether or not they have more ranks than their enemy."Let's CTRL/C CTRL/V as you pretended you would do, and
cross "have more ranks than its enemy". Result:
"Stubborn units are always defeated, having more ranks than its enemy, taking a Break test without applying the difference in the combat result scores, whether or not they have more ranks than their enemy." 
But instead of doing what you announced, this is what you acknowledged:
- to cross "have more ranks than their enemy"
-
"to allow for "Stubborn units" to be the subject" in replacement of "units".
Result:
“If a defeated stubborn unit has more ranks than its enemy, it takes a Break test without applying the difference in the combat result scores, whether or not they have more ranks than their enemy." As this sentence makes no sense, you shuffle it a little bit, and you get:
2. Stubborn units always, if defeated and have more ranks than their enemy take a Break test without applying the difference in the combat result scores, whether or not they have more ranks than their enemy.Still, despite your twists, I can only see that, according to you, to be stubborn only allows to remove
"more ranks", but not
"if defeated".
Can I take what you did as an agreement that being stubborn is equivalent to having more ranks than their enemy?
Might I faintly suspect that being defeated is not inherent to the status of being stubborn, and can only be an additional condition?
As the only specificity of stubborn units is to be always steadfast, may I dare to conclude that,
according to what you have written, being always steadfast is equivalent to having always more ranks than the enemy?When you feel ready to go further and examine whether being defeated might not be inherent to the status of being steadfast, that is probably what I would consider next.
@ Fidelis & Noght
Trying to do what you say simply does not work, you have to twist it.
Cursing GW and other W-E members, and distorting their words in a desperate attempt to prove them wrong, will not remove the flaw in your own point.
@ DonJulio
This is a rule discussion.
I'd like too to answer tit for tat against personnal attacks and deliberate distortions of statements.
But past discussions showed that it did not help, so these methods are best left to those who lack arguments.
And humour is not always well perceived, alas!

Let's see if we receive real answers, or just, let's say, "stubborn =/= steadfast, cancel all, whopdeedoo,"
or "the bold sentence tells you that units not losing combat are not steadfast, whopdeedoo!".
Or no answer at all, just a total refusal to face reality?