What are built-in pages?
Built-in pages are those provided by Spaces to perform the main functions in the Spaces Website, such as registering a new member, making a booking or paying an invoice. You cannot delete this pages as they are required for the space website to function correctly, but you can change they way they look and where each component is placed within them.
You access the code in this pages from the "Common Files" section in the template editor. Click on the "plus" icon to expand that branch of the tree and be able to see all the files.
Almost every file in this list represents a section of the space website, with some exceptions which we will see shortly. To get a description of each file, see the "Built-in" pages section of the side navigation bar.
Source: XML ! RSS