I think the boundaries of my "expertise" are context-dependent. Among academics, I have an extremely narrow expertise, and an extremely narrow area that I'm comfortable talking about and claiming any kind of authoritative knowledge of.

But when I'm teaching, I don't need the same level of "expertise" that I do as a researcher - as long as I've read the key works on the subject and done my homework as an instructor, I can be "expert" enough to teach a course on something that is quite outside my own research.

And in public settings like reddit, my "expertise" widens even further, largely because a lot of what I do here isn't so much explaining the state-of-the-art research as pointing out bullshit that is clearly wrong, or painting a basic picture of a historical subject that I'm familiar (but perhaps not expert in) in very broad strokes.

Maybe it helps that my field is pretty new and pretty small - it's easier for me to keep up with the field because there's not that much being published, and the list of "essential" works is still short enough that one person can conceivably still define themselves as an expert in "the history of childhood" (assuming we're talking about "in the western world" or "in the united states").

Language. Thankfully my Era has a lot of sources that were published (both of the "important" people and the journals and diaries of the "common" people), but it's limited by language. Worse, quality histories are limited to language yet Anglophone historiography which tends to punish the French for supporting Napoleon and demonizes Napoleon as if he is literally Hitler. As a result, I've had a very quick lesson on how to read history in Introductions and Reviews to not waste money or time.

Articles are not as bad, there are a lot of well written articles that are as close to unbiased as possible, but most of those focus on military matters.

It really is curious to see how for two hundred years, Britis historians (and worse pop historians) dominate an area of history simply because of language.

I'm in the process of learning French (and Spanish, but that's a family matter).

So, this is the best way I heard expertise in history explained.

Draw a circle that represents all of history. Draw a slice that takes up 25% of it. That's everything that you can possibly know casually.

Draw a slice inside that that takes up 10% of that one. That's what you can learn with a general history degree with a decent amount of understanding.

Then repeat with a slice thats 10% of that. Thats what a masters degree gets you.

Then repeat that with another 10% of that 10% for a PhD. That's what you are an expert in.

Something I've struggled with in terms of the boundaries of my flair is terms and language. I am very deliberate in regards to the terms I use when discussing sexuality/sexual orientation/gender identity, but it is always a challenge. When I was initially flaired, I chose "American gay and lesbian history" as the non-AIDS crisis part of my flair. Those terms were a deliberate choice on my part. I've since shifted "gay and lesbian" to "LGBTQ" which was again a deliberate choice. I think situating why I chose the first terms, and why I changed to the second provides a good example of how we draw boundaries of expertise.
Why I chose "gay and lesbian"

1. The largest part of my focus has been post-Stonewall (the Stonewall Uprising/Riots in 1969, generally used as the start date for the "modern" LGBTQ movement.) "Gay" and "lesbian" were the primary terms of the period, and throughout the 70's, 80's, and early 90's "gay and lesbian" was the standard and politically correct term of use.
2. My more specific focus is on the relationships between gay men and lesbians in life and in activism. My knowledge is largely informed by studying people who self-identified as gay or lesbian.
3. Limitations of the source material. It can be hard enough to find sources, particularly academic and secondary sources on gay and lesbian experiences. It is even more challenging to find sources on self-identified bisexuals and on trans* experiences. I initially chose to keep bi and trans experiences out of my flair because I felt I was not as knowledgeable on those identities as I was with gay and lesbian ones.

Why I've moved towards LGBTQ:

1. Expansion of my knowledge. I've made a conscious effort to broaden my knowledge beyond just self-identified gay and lesbian experiences.
2. New ways of framing history, identity, and context. "Lesbian," "gay," "bisexual," "transgender," and "queer" all have extremely complex and complicated histories. They are also all quite modern terms. Although it is problematic and anachronistic to apply them directly to historical expressions of gender and sexuality, it is natural to seek out the roots of these modern identity categories. As such we can look at the diverse kinds of same-sex relationships between self-identified women and men, the experiences of people who experienced romantic and/or sexual attraction to both genders, and a huge variety of cross-gender behaviors, identities, and experiences. Additionally it is important to remember that "heterosexual," "straight," and "cisgender" are just as much a modern concept and identity as the other terms. When we narrow a vast and complex array of gender and sexuality -based activities and identities to an assumption based on what we consider "normal" and "unmarked" today, we erase so much of our history. Erasing the history of a particular group is one of the most subtle and devastating ways of maintaining that group's marginalization. So expanding my expertise to 'LGBTQ' history signals an expanding of my interests and knowledge and a willingness to examine the intersections of gender, sex, and sexuality.

So for me, my expertise has boundaries that are changeable and also informed by these very specific and complex terms. I try to always be very deliberate about which terms I'm using, and when, but as I said earlier, it is a very real challenge to make sure that the terms I use are the terms of best fit for that particular discussion.

My "area of expertise" is mostly informed by the scope of my hobby - I belong to an antique car club that focuses on "brass era" automobiles (1906-1916). I limit myself to answering questions about automobiles from their invention until the outbreak of World War Two, and focus mostly on the American side of things. Because a lot of my knowledge comes from working on and around these cars, I'm much more comfortable discussing technical aspects - for any particularly specific questions about cultural impacts or developments related to the automobile (gas stations, highway systems, automobile clubs) I'll usually have to consult my sources - this is especially true for my background, because there is a lot of "received wisdom" among old car folks about the experiences of early car owners that is not always accurate, and I've had to unlearn a couple of "facts."

As someone considering Masters and further, Im very interested in how the area of expertise is arrived at. Is it a matter of just doggedly pursuing what you want to do or is it more a matter of going for the unexplored areas? And if the latter is it a matter of going into areas that there is a consensus need more exploration or is it a matter of finding and creating the niche yourself? For example there would be tons of guys who want to do Second World War military history, but presumably not enough spots to go around, so do people go for something very specific and niche to keep them generally in the area they want to study even if its not exactly what they want to study?

My flair is old for this sub and not altogether accurate, but it was early days before moderation tightened up salute and the sub has morphed since then.

I believe myself to be expert in method, theory, laws, and practice of Mid-Atlantic archaeology as well as in archaeological curation. I have a strong facility with and affinity for archaeology of Colonial period sites, slave sites in the upper Chesapeake, and a working knowledge of particularly ceramics, but also other material culture from the Middle Archaic forward. This working knowledge however, usually translates to the "what"'of the thing rather than the "why" of the thing. To be truly expert in archaeology you must understand both.

I find myself able to answer "how do we know what we know" questions more easily than "what do we know" unless the question is narrow enough to be in my geographic region or broad enough to encompass larger social themes. I'm also a fair hand at pointy rocks.