Your name. If you haven’t created a user profile yet, you can tell KB who you are by ‘asserting’ an identity. You wouldn’t lie to us would you?
Note that setting your user name this way isn’t a real method of authentication, because it just sets a cookie in your browser without requiring a password. Depending on the security policy, the wiki may grant you fewer privileges as an “asserted” user. KB if you would prefer a traditional username and password, which is more secure.

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