Great article.
They concluded:
Hearing aids with hollow earmolds, domes, or a large vent are suitable for listeners with near-normal low-frequency hearing and mild to moderate hearing loss of up to 70 dB HL at mid and high frequencies. Otherwise, more closed fittings are recommended.
It seems that 70dB is the loss at which the closed domes are recommended
I don’t think it can be quite so rule based. Here’s a quote from Um Bongo from awhile back.
Somewhere back in the mists of time I put together a fairly authoritative answer on this particular question as it had come up for serious debate - I tried searching for it, but I think the terms were too common.
Put simply you have two routes - Occluded and Open, though frequency compression gives you some more room at the margins, the overall approach is the same.
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Occluded: essentially stuff the biggest receiver on there, block the canal with a mould and drive enough sound to allow the 6-8Khz to be audible. Yes it can be done, but is it a good idea - possibly not for two reasons. Namely: the sensation of your ear being blocked and the subsequent overproduction of LF sounds AND the effect called the downward spread of masking, which prevails over your better hearing in the lower pitches.
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Open, smaller receiver, much more open fit and accept you just can’t hit the (probably dead) regions of the cochlear, used with a very little bit of frequency compression to bring some of the higher sounds back into play. You end up with a much kinder fitting to wear every day and perhaps miss an octave of sound that you would be otherwise overloading your entire cochlear with.
Nearly all my fittings go down the #2 route - less is more etc.
I have an open fit because I did not like the occlusion while testing in the store.
I think what the authors of the article wanted to say is that with the higher loss the benefits of the closer design are greater than the negativity of occlusion. So more pluses than the minuses given by the greater loss users to the closed design
That quote was in response to a question dealing with steep losses - it does cover the occlusion trade-off though.
I guess I was just objecting to the 70 dB cutoff. It clearly depends at what frequency/ies. If somebody had normal hearing up to 6000, but had a profound loss at 8000, I don’t think anybody would even suggest hearing aids, and certainly not a closed fit. Still highly doubtful aids would be suggested if normal to 4000 and profound at 6k and 8k. It becomes a tradeoff of how much normal hearing is one willing to give up to try to solve another problem (Would be really cool to be able to push a button in a noisy situation to turn the aids into a closed fit and activate a good speech in noise program!
Cool - dynamic venting - If you’re quick we can patent this.
My personal choice would be a smart material in the domes that gets stiffer when you put voltage through it. Link it directly to the receiver output. So you put in a very flexible dome for all people, but when you drive the receiver harder the seal is improved/orifices are closed etc. You wouldn’t make it resonate with the actual speaker frequency, but over a smoothed average. End result, less fatigue, less feedback and more venting - obvious flaw might be the action when speaking yourself, but there’s no reason that the detection of occlusion can’t happen via a mic/processor in the receiver unit.
I lke the idea! 
How about a material that sound transmission ability varies depending on frequency (lets lower frequencies through to prevent occlusion, but reflects back higher frequencies?
In my own experience I’ve found several points about a closed fit to be worth more than any sense of occlusion:
(from the 2016 review paper’s summary at end, on disadvantages of an open fit)
“Second, the interaction of the direct and amplified sounds can cause audible artifacts and can lower subjective sound quality. Third, the benefit of adaptive features such as directional microphones or noise reduction algorithms can be decreased, and the available maximum gain before feedback is reduced.”
One of the papers cited in the 2016 review is a 2006 Widex technical analysis of the effect of vent size width and length on hearing aid performance:http://content.widexpro.com/images/researchArticles/DAR84.pdf
The 2006 Widex paper covers many of the same points brought up in the 2016 review paper cited by MDB (and is one of the references for the review paper). On the most PRO side for venting is minimizing occlusion, making the sound of the user’s own voice sound pleasant to him/her. On the CON side for venting, the paper has an interesting figure showing word recognition scores drop by about 1/3 (roughly) on increasing venting, depending on what you take as the max possible word recognition score in test.
Then there is the following line from the 2006 Widex paper just to the left of Figure 4 on word recognition score as a function of vent size.
"Open-fittings reduce high frequency gain. Open fittings (or larger vent diameters), for the most part, have been used for people with a high frequency hearing loss. It should be clear from the above observations that the rationale behind this practice is to maximize “comfort” with one’s own voice and not the audibility of high frequency sounds. Indeed, an open fitting typically results in poorer high frequency audibility. The clinicians and the wearers must understand the objectives (and limitations) of open-fitting so realistic expectations can be formed. "
I found this most interesting as for me, switching from an open dome to a closed power dome, although I got the occluding effect, in return speech seems much crisper and cleaner. The 2006 paper authors also bring out the point that depending on the configuration of your ear canal, etc., there is the possibility for direct sound reaching your ear through the vents to be out-of-phase with sounds produced by the HA’s and decrease intelligibility on that basis. ~Same points as made in concluding summary for 2016 review paper cited by MDB as OP in this thread and quoted in part at the start of my post here.
Edit_Update: On checking again, the direct link to the Widex Pro site for the 2006 technical paper on vent size now works again. I had complained to Widex that the link was broken and their response, from a Widex website administrator, was just to ask how I came by the link in the first place(?!). I pointed out that the other versions of the document were available on the Internet, that the paper was cited in a review posted online in the NIH National Library of Medicine and if Widex was going to restrict access to such useful and informative information that would prejudice me against Widex in future hearing aid purchases. Don’t know if that dialog had anything to do with the link being restored but ReSound in allowing the general public to view the contents of their PRO site sets a standard to match. Online Training
End_of_Latest_Edit_Update
The link I cite above seems to no longer work (Widex locked the door to non-providers?). Here is a link that works as of this posting:
Edit_Update: A copy of the paper to download is also available in less colorful, less well laid out form from Hearing Review, the original publication source: http://www.hearingreview.com/2006/02/fitting-tips-how-do-vents-affect-hearing-aid-performance/
(Google Chrome warns that this site is not secure as it does not implement HTTPS: communication)
As of 11/22/2021, I found that the Widex article can also be downloaded from the following link: https://pdf4pro.com/cdn/fitting-tips-how-do-vents-affect-hearing-aid-performance-597ea5.pdf
Proceed at your own risk! The hearingreview.com version is not very satisfying as the graphical figures are terribly small and difficult to read. It’s too bad the original PDF version no longer seems to be available on the Widex Pro website. The graphics can be viewed in high-resolution detail and the paper is overall much more nicely formatted than the hearingreview.com version.