In 4.5 four new fields have been added to the Host Settings menu. The first two additions provide useful information. The Relative path field displays the relative location of the application in relation to the root of the site (for sites running in the root, this will be blank) and the Physical path field displays the physical location of the site root on the server.
The next field, Permissions, utilises a new SecurityPolicy class to determine if a site supports key code access security permissions. Many people use shared hosting setup's for their portals, and in a lot of cases these hosting providers will be running in a partial trust scenario (usually Medium trust), and have disabled potential dangerous settings. However, whilst this does provide a more secure sandbox than running under high or full trust, there are valid use cases for these permissions e.g. modules that attempt to use HttpWebRequest to download content or post to other URL's.
To help users diagnose any problems, this field will currently indicate if the site is running under a policy that supports WebPermission or ReflectionPermission access (in the screenshot below my site is running under reduced trust so doesn't support ReflectionPermission, but the hosting provider has enabled WebPermission). Module developers who want to add code that relies on either of these permissions can use the SecurityPolicy.HasRelectionPermission and SecurityPolicy.HasWebPermission functions to check if a site can support the functionality.
Note: For more details on code access security and it's implications, you can read the DotNetNuke medium trust document here.
The final field, GUID, is a unique field that is generated for the host and for each individual site. It was added to allow commerical module developers to use them to generate license keys for their products, and was made visible to support cases where site owners need to provide it to vendors who can then generate a unique licencing key for their product(s).