So what is it? Single Copy XPage Design is a new feature in 852 that allows for XPage resources to be shared. XPages design elements (XPages, Custom Controls, CSS, client/server JS libraries, themes) are stored in one database and then other databases can point to this common "SCXD" database through a new database property available in Designer (in the Performance section on the XPages tab in the Application Properties). At runtime, XPages will read this information and, if it is not empty, will then use the design elements from the "SCXD" database instead of the current database. So effectively you have UI in one database and data in another.