http://dauntzs2oj7q36bjd5ttdakaebh6l527dttze4tu363q3etsiulylhad.onion/faq.html
Authentication in many cases is not as simple as just logging in to your user account, you may need to meet other arbitrary criteria, which is defined by the service provider. The service is able to define different tiers of access to their mirrors, all with different requirements from you. This will vary per service with some having more complex rules applied for different sets of mirrors or some may also offer additional mirrors with more "entry level"...