Base, Debug, and Release are the three default build configurations. Base acts as a base set of option values that is used in all the configurations you subsequently create. The Debug configuration enables optimization and debugging, as well as setting specific syntax options. The Release configuration doesn't produce symbolic debugging information as well as the code is not generated for TRACE and ASSERT calls, meaning the size of your executable is reduced. 

You can change option values in any configuration, including Base. You can delete the Debug and Release configurations, but you cannot delete the Base configuration.  

You can change the options for the three default configurations, and you can create and name your own custom build configurations on numerous build-related pages of the ProjectOptions dialog box.

Every project in RAD Studio has an active build configuration associated with it, as well as any number of other inactive build configurations that you have created.  

The active build configuration is used in Compile and Build commands for the project. Use the Build Configuration Manager to specify the active configuration for a selected project or project group (choose ProjectConfiguration Manager).

MSBuild Overview 

Creating Named Build Configurations (Delphi) 

Applying the Active Build Configuration