What is the purpose of BASE.xml file located at C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\15\TEMPLATE\XML\BASE.xml
It is used for defining Sharepoint base list types (Document library etc).
Found the current MSDN documentation
Here is the snippet regarding Base.xml:
BASE.XML: Provides the schemas used in creating the Lists, Documents, and UserInfo tables in the database when a new site is provisioned.
The link also provides documentation for all of the other Major CAML files.
Good to know:
With the exception of DocIcon.xml, Microsoft does not support modifying any of the built-in files that are installed with SharePoint Foundation. In most cases, you can supplement or override the effects of these files with files of your own.