There is only one file type that uses the .XCDATAMODELD extension:
Xcode Core Data Model File
Developer file created by Xcode, a free software IDE developed by Apple; created when the developer specifies that Core Data will be used for the project; saves an .XCDATAMODEL (without a "D") file as well as other files.
You can view the contents of an XCDATAMODELD file by right-clicking it and choosing "Show Package Contents."
NOTE: Core Data is a persistence framework that allows developers to interact with underlying data sources using a high-level data model.
Updated: April 21, 2012