Excluding a page (and its sub-pages) from referencing means that search engine spiders cannot index it.
A page excluded from search engine listings will be removed from the sitemap.xml file and its url added to the robots.txt file.
The button is available in the ribbon in the "Page" tab:
Once the page has been selected (via the sitemap tool), clicking on the "Out of referencing" button opens the following pop-up window, requesting confirmation from the user.
Clicking "Yes" excludes the selected page (and its subpages) from the listing.
When a page is excluded from referencing, the icon of the button is modified, so that you can quickly see that the page is excluded from referencing.
To perform the reverse operation (include a page that has been excluded), click on the "Out of listing" button again to open this pop-up and ask the user for confirmation.
Clicking "Yes" includes the page (and its subpages) in the listing.