Disambiguation pages are tagged with __DISAMBIG__ so that they're easier to work with; for example, tagging a page allows the software to easily tell whether a page is a disambiguation page without having to parse the page, and have a consistent API to tell clients this information.
Almost always, this tag is put into a disambiguation template, so even the most experienced of editors aren't aware that the tag exists because they never see it and never have to add it. Nevertheless, there is a button in the page settings box in the visual editor to add the tag. There is no tooltip or other explanation given to the user as to why it's there or what it does. There probably should be.
I'll admit 2 things:
one is that I have never used the magic word to create/tag a disambig page. Interestingly, my colleague from en.wp said the same thing,
The second is that I would have loved to figure out what the option actually did via the (I) icon, but there isn't one there, and I don't even know if you ever planned to have one in the first place. TY!