Your registration options in XNAT will be dictated by the type of XNAT you plan to run. Are you setting up a publicly-accessible data repository like XNAT Central? Or a tightly-controlled data gathering and QC application for a multi-site study, like IntraDB for the Human Connectome Project? Your PI's business goals will need to be reflected in these registration options.
Require Email Verification to Register?
This determines whether users will need to verify the email address that they enter when registering before being able to use the site. If set to 'Required', then newly registered users will receive an email with a link they can use to verify that the email they registered with is an email they have access to. If set to 'Not Required', then no verification emails will be sent and users will not be required to verify their email before using their account. The preference below this one, Email Verification Message, is the text of the email that gets sent to the user that just registered.
Email Verification Message
This is the contents of the email that gets sent to newly registered users when the previous preference, 'Require Email Verification to Register?', is set to 'Required'. HTML tags can be used in formatting this message if desired. In addition, this message can be personalized by including FULL_NAME, which will be replaced by the user's first and last name. In addition, any instances of VERIFICATION_URL will be replaced by the URL that the user needs to use to verify their email. This URL will be slightly different for each user.
Auto-enable User Registration?
This determines whether new user accounts will be enabled automatically when the user registers. If set to 'Auto-enable', users will be able to access the site and view public projects immediately after registering (and after verifying their email if email verification is required). If set to 'Do Not Auto-enable', then the site administrator will have to manually enable newly registered users before they are able to use the site.
Auto-enable with Project Access Request?
This determines whether users who register in the course of accepting project access requests should be treated differently to users who register normally. If set to 'Auto-enable', user accounts created when accepting project access requests will always be enabled (and verified) automatically. If set to 'Do Not Auto-enable', user accounts will only be enabled automatically if "Auto-enable User Registration?" is true. So this setting determines whether project access requests should override how user registration is normally handled.
Allow User Comments on Registration?
This determines whether there will be a text box on the user registration page enabling users to enter comments when they register. There comments can then be used by the site admin in determining whether to enable the new user's account or determining what to give the user access to. If set to 'Allow', a comment text box will show up on registration and any comments entered will be sent to the site administrator when notifying them of the new user registration. If set to 'Do Not Allow', then there will not be any text box in which users can enter comments when registering.