I believe it’s used in the SAML auth provider if that’s any help to you:
https://github.com/discourse/discourse-saml/blob/172cc6c3094cf78036a782e6c22da70e5c722f34/plugin.rb#L22
https://github.com/discourse/discourse-saml/blob/172cc6c3094cf78036a782e6c22da70e5c722f34/plugin.rb#L124