Excluding a page (and its sub-pages) from referencing means that search engine spiders cannot index it.
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.
From version 4.3 onwards, it is possible to select the root of the sitemap. Clicking on the "Exclude from referencing" button allows you to exclude the entire site from referencing at once.
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.