What does SQL Server's Error Log actually log?

If you have ever played around with the error_reported Extended Event, then it becomes very obvious that the SQL Server Error Log only logs a tiny fraction of the errors that are thrown by code running on your server. Indeed, many things that it logs are explicit about not even being errors. So, broadly speaking, what determines if an error is worthy of inclusion in the Error Log? Is there some list of errors that qualify? Or is it determined based on some criteria?