The Preferences page under settings on the admin navigator controls login options. They are as follows.
When checked, a site user can use their email or username in the username field.
When checked, non-content managers can login with a password if their account does not include a password.
When checked, the standard site login includes a form to email a person's username and password to their email address.
When checked. a returning user will use the same user account as the visit from the computer they are using. They will not be logged in so they can not access blocked content, but they will be tracked, and personalization on the pages will work.
When checked a returning user will be logged into the same user account the last visit from the computer they are using.
When checked, you can have multiple accounts with the same username. This is not a secure configuration, but can be used to meet specific username/password configuration requirements of some organizations.
When checked, this site property can be used to control add-ons that might have signup forms. For instance, the Contensive blog addon has an option to only allow comments if your registered. in this case, it allows visitors to register only if this check-box is checked.
if you have an alternate log-in form, paste it into an add-on and select it here. The add-on will be used internally to block content. It will be called to create the form, and again at the beginning of the next page if the form is submitted (to perform the log-in function)