I spent the last 10+ years doing hardware/software interfaces for new cell phones. With a little specialization on the manufacturing process. I did this for two different companies, and was considered an expert in both of them.
No, I don’t know how to share a contact with a friend. Yes, I do understand why some phones have removable batteries and some don’t.
Sailing, with sails only, from the Port of
Catoosa to the Mississippi River.
There is a sliding scale for the term expert. In court the Judge decides if I am an expert witness. Actual qualifications may not have bearing.
So somtimes I am a data forensics expert.
Drooling.
No. I am mediocre at everything I do.
Took me a long time to realise that that’s enough for most situations.
Optical kerning.
Seriously, optical kerning.
This. Exactly.
I’m an expert at getting patent applications filed around the world and at keeping them on track (know what needs to be done when, what papers and what fees need to be submitted, etc.). I’ve been doing it for 30 years and I can tell I’m an expert because all the attorneys I’ve worked for have come to me (I’m not an attorney) to get it done for them and/or to answer their complicated questions about it, and I’ve seen enough things go wrong that I know what to do about it when it happens.
Gosh, I need to get back into it. (Currently unemployed.)
My lung capacity is something like 4-5 standard deviations to the right. So I was told when I was having it measured. Does that count for anything? Just call me awesome lungs.
I’m an expert in a certain area of digital design, enough so to be the Program Chair of the biggest conference in it, and on the editorial board of a journal, and on lots of panels. Not the top expert, but an expert.
When I was in grad school Springer published a book with a collection of papers with one of mine (and others in our department also) and the ad for it said that it was composed of papers by leading experts, so q.e.d.
Indexing books, both theory and practice. Ask me a question about it and I can answer it for you.
Yes.
Chess coach.
I’ve been doing it for 40 years (the last 25 as my full-time job.)
One of my students won the World U-18 Championship. 
Which book is the most difficult to assign to one category , or another ? Does one book in particular keep getting found at two different places/numbers ?
The mechanics of the auction processes that establish the opening and closing prices of stocks on the New York Stock Exchange and NASDAQ.
Not anything I can think of; but I’m more knowledgable than many people on a couple of things.
I was given the MVP award for Microsoft Access for 4 years in a row. I also earned a living with Access databases and taught it at a local college. I considered myself an expert in that area, but have left my skills lapse the last couple of years as I now work with Oracle databases.
I was an aircraft mechanic for 20+ years and even taught aircraft maintenance. I was pretty much of an expert in that field.
There were a few times over the years where I had a large amount of knowledge on a very narrow subject matter. Often that’s what an expert is.
But mostly I’m a generalist. While I was a professional aircraft mechanic, I also work on cars and bicycles to this day. Same with computers. I’m a database administrator; however, I can also do other things in the computer field.
I generally accept someone as an expert if others will uniquely pay them for their time, and/or they are skilled or knowledgeable enough to potentially make a living from it (where it would apply).
There are exceptions, but this generally weeds out the people who know more than a casual person, and generally exist as experienced persons, but not quite professionals.
I am competent to “commercial expert” at a number of things, but I’m not sure that’s the OP’s question.
I was at one time the acknowledged world expert on a… cultural issue. I have since been surpassed by a small handful of people, all colleagues and proteges.
I am perhaps the world expert on another cultural issue except for the remaining cadre of people who actually formed it, most of whom are now no longer involved.
I have two projects, one very active and one somewhat stalled, that represent top-tier expertise but are not yet complete enough to have influenced anyone.
Oddly enough, none of these have anything to do with my day-to-day professional work, although the first one was close to a profession for about ten years. It all meshes, though.
I spent 30 years as a metrologist. I measure things. I got pretty good at it. Kind of a narrow field.