Config transforms

https://msdn.microsoft.com/en-us/library/dd465318(v=vs.100).aspx

When you deploy a Web site, you often want some settings in the deployed application's Web.config file to be different from the development Web.config file. For example, you might want to disable debug options and change connection strings so that they point to different databases. This topic explains how to set up a Web.config transform file that is applied automatically during deployment in order to make changes to the deployed versions of Web.config files.

Web.config transforms are part of a broader group of settings that you can configure to automate the deployment process. For information about the tasks that are involved in setting up automated deployment, see the following topics.

To specify the changes that you want to be make in Web.config files, you use transform files. A transform file is associated with a build configuration. By default, Visual Studio creates Debug and Release build configurations. You can also create custom build configurations.

For TDS:

https://www.hhogdev.com/blog/2015/december/config-transforms.aspx