When and where does underscores do what?

I see underscores used as a prefixes on

some, but not all, of the tags in /admin/items, e g _xxx
some, but not all, of the keys in /admin/settings, e g _onify_domain (all of the keys with the underscore prefix has the role "custom", but there are also keys without that has the role "custom")
some, but not all, of the keys in /admin/configuration/strings

Searching within readme support shows just one hit, which is not related to this:

Searching the forum by Google shows just one hit (though another one), which is not related either:

(We already came to the conclusion that Readme searches only within the titles, but I don't know why Google don't index the page https://support.onify.co/changelog/onify-hub-api-2140-released#2140 found in the Readme search? You might wanna look into that if it is meant to be a searchable support forum where you can find the answers yourself.)

So my question is - what are the underscores for?
It seems prefixing a tag in items hides them from showing up in /search, but why is it needed in some settings? It would be good to get a full documentation of when underscores does what and if it is used different in theses three places (or more)? 😀