XML-Document-Transform (xdt) is a technique used in Microsoft Visual Studio for modifying Web.config files, typically for debug and release deployment scenarios.
XML-Document-Transform (xdt) is based on a principle of base config file (App.config or Web.config) and one or more Transformation files that specify how the original config file should be changed upon deployment.
XDT reduces redundancy of config files by storing most of configuration data in a base config file, while Transformation files only contain differences that are required for each particular deployment scenario.