Short version:
It's a specific brand of barrel designed to shoot softly and give you the soft shooting characteristics of a rifle length 18" barrel by using the not very common (and also not very much agreed upon dimensionally) intermediate length gas system to give you only slightly higher port pressure with similar dwell time to an 18". And to mitigate port pressure, they gas port size is reduced.
Oh also I left out of my previous list of things, a 1:8 twist .223 wylde chambered barrel is the right thing.
TLDR version.
Most AR barrels have the gas port drilled to over gas the gun. This is generally more reliable in more situations with more parts, but it means you bleed off more gas and beat things up more. Additionally, the shorter the gas system, the closer the port is to the chamber, and the higher pressure you get at the gas port. The farther the gas port is from the muzzle of the barrel, the longer that gas impulse lasts. Too low a pressure and too little dwell time, and the bolt won't unlock reliably. Too much pressure and dwell time, and it'll probably work, but you shorten the life of the components and slam the carrier group and buffer around a lot. Oddly enough, and confusing for a lot of builders, being overgassed and undergassed have the exact same failure mode for different reasons. Got a little too much gas and you get FTFs because the bolt comes back faster than the magazine can shive a round up to be caught. A little too little and you get an FTF from short stroking. Way too much and you get FTEs I believe because the case is still being expanded by gas pressure when the bolt unlocks. Way too little gas and you get FTEs because the brass never clears the chamber and just gets reinserted.
That's probably way too much. If you want to know more about AR gas systems, this is one of the more informative articles: http://www.ar15barrels.com/prod/operation.shtml
Anyway, as 3 gun progressed two main factors made lighter guns better. 1) more popular meant more matches which meant 300+ yard shots got less common in the average shooters match season. 2) true 1x LPVOs came to dominate and you could have your cake and it eat too provided you didn't mind slapping 2lbs on top of your rifle.
Because that's a lot of weight out of an 8lb budget some other things became more popular: reduced gas system with lightweight buffers and carriers, lighter barrels, and lighter forearms. Mostly because you can retain a lot of the functionality while losing a lot of weight compared to other things.
18" barrels with rifle length gas systems became popular because you could slap most any 20" barrel in a lathe, cut and re-crown it, and it'd work. Even though a 16" barrel will get the job done very similarly to an 18" at the ranges you will encounter in most matches, it won't work with rifle length gas unless you get very specific about port size, ammo, ambient temp, etc.
So after a while, it became pretty accepted that for an 18" barrel, a 3gun profile would give you the best combo of weight an accuracy. That's basically a 0.75" profile back to the chamber area except for the gas block flange. Stretch precision ran with this profile, chopped it to 16" then made the gas system as long as they could without opening up the port too much, and then gave it some minimal fluting. And you have a soft shooting 28oz barrel.
It's closest competitor is probably the ballistic advantage 17.7" hanson profile at 31 ounces, the ar15perfromance scout at 28oz, the faxon flame fluted 16" match series barrel at 28oz, the 16 " faxon gunner at 21.28oz, the 18" faxon gunner at 24.62oz, oding works also has similar gunner-like 16" as well as a 31oz 3gun profile at 31oz.
(There are lighter pencil barrels, but they tend to be more whippy and drift with heat soak. The gunners are pretty much at the practical limit of tolerating that in most 3 gun matches).
but all of those are mid length except the 18" gunner which is rifle length but has a small gas block shoulder that means you can't use most adjustable gas blocks. The odinworks has non standard spacing to the gas hole and you need to use their block unless they got smart and made it normal. If you want lower gas pressure you and normal gas blocks, you are shooing a limited number of 18" barrels on the market.
It's pretty nit-picky and pretty niche but I'm happy with mine. I don't know that I'd do it again and will probably just get a 31oz 18" when I have to replace it.