"D-ring" has the same meaning as the word "semi-ring". This is a method of fixing helmets, fastening clothes and other equipment using specially shaped "D" rings (for example, see ARAI's modern technology for motorcycle helmets, which guarantees that in the event of any hair-raising fall the victim will still have his helmet on their head).
Why prefer "D" rings/semi-rings over other types of clamping?
Because they are often much more reliable than all kinds of modern quick-release buckles, as with them (for example, after exposure to extreme weather and therefore also the potential for corrosion) reliability decreases over time (their miniature mechanism in winter with ice around can easily partially melt on the body and at all then not fulfill its function, even if it appears to be closed). Not to mention that the quick-release buckle is the only one-size-fits-all, so it may press many people under the chin.