XDT Transformation Tool
XDT Transformation tool is a command tool based on XDT (web.config) transform engine
, which allows you to run XDT transformation on any XML files.
Scott Guthrie: "In most real-world deployment scenarios, the web.config file you use for development is different than the one you use for production deployment. Typically you want to change environment settings like database connection-strings, making sure debug is turned off, and enabling custom errors so that end-users (and hackers) don’t see the internals of your application."
If you want to know more about XDT Transformation Syntax, follow this link Deployment Web Application Project
. To know how to start using XDT Transformation Tool follow this link Documentation
Also you can find posts on Russian Config Transformation Tool