But it seems true that non-Lutherans have OTHER definitions....and so it's not surprising that there is confusion on this.
And I think people unlearned in epistemology simply have no clue what "rule" and "norm" mean. These are words from philosophy, specifically from epistemology - the issue of what is true, what is credible and how to determine that.
In Lutheranism, the issue here is very specific: There ARE disagreements among the world's Christians... in both teachings and practice. Sometimes these are in dogmatic and definitive issues. Sometimes they are highly divisive. SO, the issue becomes: Who is correct? Which of the teachings is correct?
I've never liked the term "Sola Scriptura" (Luther himself chose this.... likely to go along with his other "sola" statements). "The Rule of Scripture" is the title I tend to use.
In epistemology, addressing this involves three aspects:
1 Accountability. All the positions are POTENTIALLY wrong, no one gets a "free pass" from accountability. This is often a problem in Christianity since there are those who demand a unique exemption - insisting all OTHERS could be wrong but there is ONE unique, individual exception among all the teachers on the planet, and that ONE just happens to be self ("self" here can be an individual person or church or denomination). The Lutheran position rejects self exempting self uniquely from accountability, and this is why the Catholic Church so rejected Sola Scriptura since it itself insists that it itself cannot be wrong (under certain conditions). Catholicism "opts out" at this point. So do the cults and some Protestants who claim special status as uniquely, individually lead and protected by God. Those who reject this whole process do so for THIS reason, they need to exempt ONE from accountability (and that's always self).
2. Rule. All disciplines tend to embrace some rule.... something objective and knowable that can be used by all disputing parties to determine which view aligns with this. In the Rule of Law, civil disputes are decided according to the words of the law; if a cop says I was speeding and I say I was not, we both agree to look at that big square sign by the side of the road with the big black numerals of "50." In physics, we often use repeatable laborative evidence and mathematics. Let's say Tpm and Jim hire Bob the Builder to construct a 6 foot fense on their property line between their houses. Bob is done and says the fence is 6 feet tall but Jim says it's only 5 feet tall. Perhaps Tom and Jim and Bob all agree to use a Sears Measuring Tape to see if Bob's claim is true. In this case, the Sears Measuring Tape is the Rule, the norm (or as it is called in epistemology, the "norma normans" - literally, the norm that norms. THIS IS THE SINGULAR ISSUE IN SOLA SCRIPTURA - specifically WHAT is the best rule. Yes, it depends on #1 above (all disputing views are accountable) but the issue is only WHAT should serve as the Rule or Norm or Canon ("canon" refers to the measuring stick used in construction by the Greeks - the measuring tape). And in epistemology, the more objective, the more universally accepted, the more "outside" all parties in dispute, the better. Sola Scripture embraces that SCRIPTURE is to so serve.
3. Arbitration. There then is the issue of determining IF the teaching/claim/view "measures up" to the "measuring stick" (the Rule, the Canon)? Tom and Jim told up that measuring tape.... and it says 72 inches.... SO does the fence "measure up" or not? That's arbitration. In the Rule of Law, a court decides if the behavior "measures up" to the Rule of Law. In physics, scientific journals work to see if the theory or view "measures up" to the observable evidence and the math. Now, admittedly, this remains a problem. Since the demise of Ecumenical Councils (the last of the 7 ended in 800 AD) there simply is no "court" to which all Christianity is bound, there is no "court" at all for the whole of Christianity. Individual denominations often have some means of final arbitration but nothing beyond the denomination and this is perhaps part of the reason why Christianity has spun so many denominations ... even before 800 since not everyone accepted all 7 Ecumenical Councils. When people note that Sola Scriptura doesn't always RESOLVE disputes among the world's 2;.2 billion people, they are correct and this is why. The case can be made..... perhaps very clearly and boldly... but the "jury" tends to be us and the virdict isn't always the same. But does the lack of this third step make the previous 2 meaningless? I don't think so. As limited as this is in practice, any look at the plethora of cults (ALL of which fundamentally reject Sola Scriptura), the absurdities of liberalism (which also rejects Scripture as the Rule), the craziness of what we often see on the internet (all obviously rejecting Sola Scriptura since clearly the words of Scripture are obviously deleted and replaced by the new, wierd "interpretation" of the heretic). The vast majority of Christianity embraces the Councils, the Creeds and an enormous body of teachings precisely because we consider all SUBJECT to the words of Scripture and not above such.... most Christians are "people of the book."
Other things are NOT ignored! Indeed, they are often essential in the third step, the arbitration. How has CHRISTIANITY understood the verse(s) we are looking to as the Rule, Norm, Canon, norma normans? TRADITION plays a critical role here.... we will look to the Seven Ecumenical Councils, the Three Ecumenical Creeds, the universal/ecumenical faith of Christians over the centuries. Now this MOSTLY has to do with hermaneutics - how to understand/interpret the Scriptures, but it's not limited to such (indeed, what IS Scripture is a matter of Tradition). Again, this is always true in epistemology. In the Rule of Law, the court will look at how this law has been understood and applied in the past. Such CAN be wrong... but which is taken into account.
That's the ENTIRELY of "Sola Scriptura." I will admit most ALSO argue that Scripture is not only the sole Rule/Canon BUT ALSO the sole source of doctrine. Actually, Sola Scriptura does not say that, but I will admit it's a natural consequence of it: If a teaching isn't found in Scripture, it cannot be normed as true by such.