Your site setup 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.
The ID used to refer to this site (also used to generate database ids). The Site ID must start with a letter and contain only letters, numbers and underscores. It should be a short, one-word name or acronym which describes your site. No spaces or non-alphanumeric characters. By default this is set to 'XNAT', but you will likely want to change this to the name of your particular XNAT instance (e.g. 'ConnectomeDB' for the Human Connectome Project public site). This Site ID will appear throughout the site, such as on the page users are directed to after login which informs them how many projects, subjects, and imaging sessions the site has (e.g. SITE_ID currently contains 2 projects, 3 subjects, and 4 imaging sessions.).
This is the URL at which users should access your site. It should be the full URL, including 'http' or 'https'. This URL is the one provided to users in emails, and other external links. This should be a functional address (i.e. if the user pasted this address in their web browser, they should come to the site). localhost only works if the web browser is located on the same machine. You are required to guarantee that this address is functional for reaching the site.
This controls what site description shows up on the login page. You have the option of either entering the text of the site description directly using Markdown or specifying the path to the Velocity template file which contains what you want to show up for the site description. If you want to use a template file for the description, you should select the 'Page' radio button and then enter, in the text field below the radio button, the relative path to the site description template (e.g. '/screens/site_description.vm). If you want to enter the description directly, you should select the 'Text (Markdown)' radio button and then enter, in the text box below the radio button, the site description using Markdown.
Site Login Landing
This is the relative path (e.g. '/screens/QuickSearch.vm) to the Velocity template page that you want users to land on upon logging in.
This is the relative path (e.g. '/screens/QuickSearch.vm) to the Velocity template page that you want users to land on when they click the XNAT logo in the menu bar.