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.