The Structure of Trauma
The image that one might get about trauma based only on the criteria for PTSD is that it's mainly about symptoms of emotional dysregulation. I think this is too narrow, especially for trauma smells. One view that I like says that trauma is something like an exaggerated and overly restrictive response to real fear, horror, and grief.
It seems like in general, mammals, at least, learn on a kind of "emotional" basis, as in, following gradients of valence and emotion. This often follows some thread of curiosity, satisfaction, or pleasure, or their opposites. This can be gentle and slight, or can also take the shape of very large updates in response to acute, intense valence, whether positive or negative. IIRC both of these are observed pretty straightforwardly in primates, dogs, rats, etc. Perhaps, some people are capable of learning in a "neutral," "technical," or "clinical" way from experiences that for others are strongly valenced, but descriptively this kind of emotional imprinting is how things usually go for most people, for experiences of extreme danger, joy, etc.
Organisms in general also need to be highly sample-efficient when it comes to danger: you don't get many chances to almost die, or almost lose your leg, and you want to really learn your lesson from near misses. Humans can also transmit various kinds of dangers culturally, besides the much slower and coarser transmission of genetically encoded instincts that lots of species have. Still, cultural transmission is generally pretty low-resolution compared to the full reality of some threat, and even if people have internalized that something is dangerous, when they actually suffer some harm they likely won't be "prepared" for it emotionally.
So then, one updates hard, and often one both over-updates, and updates in a way that becomes very difficult to roll back.
Besides this, there is also so-called "negative trauma," basically trauma that occurs due to the absence of some experience. The prototypical case here would be childhood neglect. Now, "neglect" usually correlates with, and often has a connotation of, other kinds of adverse experience, but there seems to be a real effect where a child's environment is not overtly abusive, but there's an absence of parental warmth and care which leads to both "pychodevelopmental antrophy" as well as patterns of negative emotion etc. more stereotypical of trauma.
Blending with the view
Besides just for sample efficiency, updates about negatively valenced outcomes probably "ought" to influence our feelings and ways of seeing. While spirituality has some bearing on this, under normal circumstances it's going to be something like "appropriate" to temper one's sense of rightness, comfort, and emotional safety with respect to the real circumstances in the world. Barring the far reaches of spiritual attainment, if you know that your brother is suffering, or that you might lose your job, this probably should be integrated into your affective sense of the world. Again, descriptively, this is usually how things go.
There's a tendency for these emotional updates in response to traumatic experiences to get deeply ingrained in the default ways of seeing, often in ways that become all-but-invisible. Reports from meditators, who often have richer introspective access than most, often go something like, "yeah it just felt like the world is fundamentally horrible and unsafe, and that colored everything I saw and interacted with"—and many such variations. Again, these reports can come from survivors of genocide, but also (and sometimes with similar language) from those who've experienced totally mundane personal tragedy, familial estrangement, etc. A very stereotyped example here comes from victims of bullying over many years, who report that their experience of being low-status, shameful, etc. seemed to be simply "part of the world," or so much a part of the world that it was invisible until the burden sloughd off.
I'll refer this kind of phenomenon as cases where trauma is highly "blended with the view."
PTSD proper is partly also about when a person sort of keeps "triggering themselves," but that's less what I'm talking about here. There also seems to be something especially pernicious about negative valence, where it's self-sustaining in a way that positive valence generally isn't. Maybe, we can view awakening as an experience that's so positive that one never needs to feel bad again, but this at least isn't what happens to most people when they fall in love, or win the lottery, or whatever.
Emotional/Psychological Metabolism
Related to all of this is the concept of metabolism. For most kinds of negative experiences, and even sometimes for very intense ones, there's a variety of ways that the experience/emotions/"content" etc. can be integrated. Two tentative glosses on "integration" here might be: a. robustly and maturely understood, and weighed appropriately with respect to the rest of one's values, and b. no longer acting out a traumatized, injured response, having a mature relationship to the reality of some experience.
So, let's say, a child experiences a let-down and is utterly distraught about this, but with time comes to a more mature relationship with success and failure, at least in degrees. Not to make a blithe comparison, but probably the same is possible with respect to loss, safety, humiliation, etc. There are familiar stereotyped reports about "learning from" the loss of a loved one, of a kind of maturity and richness and flowering that comes from it, or through it, or whatever.
Part of the experience of metabolism, in the caricature both of childhood let-downs and of gut-wrenching grief, involves some intense emotionality, tears, "catharsis," etc. This accords with the reports I generally buy, whether anecdotes from joe-off-the-street, meditators, and from therapy manuals. It's salient here to note that adults, and stereotypically, especially men, often have lost this kind of catharsis as a regular feature of their lives. Perhaps, sometimes this can be because they're already "highly integrated" in this domain, but from my perspective this is an indicative trauma smell.
To be clear, a lot of what looks to me like "correct" or "robust" metabolism happens on its own, and is not necessarily loudly emotive at all. There's some cluster of evidence that trauma, and maybe especially "small-t" trauma has more to do with not feeling safe socially in a negative experience. I'll discuss more about subtler and more unusual kinds of metabolism.
There are very common reports of low-grade chronic anxiety, malaise, ennui, and even gross psycho-social dysfunction "unraveling" immediately causally due to intense catharsis. Just the same, reports of healing experiences from both "classical" and "small-t" trauma do often actually look pretty much like stereotypical cathartic metabolism as well. There are however reports of kinds of very advanced metabolism that are "rarified," and might be silent or invisible from the outside.
"Correct" metabolism
See also some discussion of correctness, in general.
This all leaves out what the "goal" or "resolution" or "end state" of metabolism ought to be. This turns out to be a larger question, which I discuss as it relates to psychology broadly, spiritual practice in particular, and meaning/value in general later. Still, I'll discuss what some of the considerations are, as it informs what I consider to be trauma smells.
First, there exists real suffering, violence, and injustice, besides which at least mortality seems to be basically inviolable. Most people are by default only sort of vaguely aware of the reality of suffering that they haven't experienced, and it seems to be difficult not to come away disturbed from facing horrors up close. Even very "personal" trauma of humiliation and abuse might be less profoundly disturbing just to witness but is still quite [distorting] to actually receive.
It's obviously not sufficient to say "well, those who haven't experienced such suffering represent some kind of truer untraumatized state"—there's real suffering which such a person hasn't really updated on. It seems like we want some notion of "maturity" or "integration," something to the effect of "wisdom despite, or through, suffering." So the question is sort of, what does maturity look like, and what is "false" maturity? (Or perhaps, trauma is just a ratchet, and you can only be dirtied or injured by it.)
A traumatized person has in some sense a thesis about what is safe to do, think, or feel. Agnostic of whether this is "correct" or "appopriate," brodly speaking it's in any case an adaptation to some adverse experience. An integrated perspective has to meaningfully take into account the full reality of the adverse experience in the first place, and not just by "faking it" and trying to say magic words which are not existentially real for you, nor by dissociating, or by just ignoring the subject.
So again, mortality. As far as I can tell, you and I are both going to die. If you learn your death is imminent, just learning that will probably be quite intense and painful, but you ostensibly already know that's coming. Have you faced that? What would it mean to have faced it? There are lots of spiritual/existential perspectives about death which largely look wrong to me. Again, I'll discuss later what a "correct" perspective would have to include here.