In the description for the fallback logo format when certain sizes aren’t provided, it’s written that large_icon will be used in nothing more specific is provided.
But what is large_icon? It’s not specified anywhere on the page which one it is.
I’m not sure what admin view or keyword result you’re viewing here, but if you go to All Site Settings → Branding you should see the Large icon setting in there:
I’m in Appearance → Logo, it’s in the texts that appear in that admin screen.
Would be worth replacing the string name by “human language”?
I can see it when I go through Site Settings → Branding, and it has been “set” by something I uploaded in the Appearance → Logo screen, although that screen does not list it as such.
For me this is typically the kind of “little thing” that makes the admin interface confusing for the first-time Discourse user, by the way.
I hadn’t noticed before, but I also have the same file appearing as Large icon in one place and Square icon in another. Now I’m not sure if I uploaded them separately, or if one was derived from the other.
It is confusing that the contents of All Site Settings → Branding and Appearance → Logo overlap partially but aren’t named & described consistently. Maybe one section of Logo and Branding would be easier to manage. 
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I think Appearance → Logo was added when many settings were grouped into config pages and made directly accessible in the admin sidebar. So, I think this is kind of the place where it should be easier for admins to configure that because it’s a separate place compared to it being hidden in all site settings. But in the end, it just looks a little different, but you edit the same site settings.
I reported before that the hints on which size each logo should be differ between site settings and config page. This also makes it more confusing.