docs/user-manual/build-system/cmake-targets.md
The build system supplies the ability to register extra targets that allow the user to register new functionality to the build system. For example, the user may wish to create a build target that counts the total number of files in the system. This would be the place to register such a target and have it available to the build system.
Targets are applied both at the global scope and per-module scope. Thus each target can provide a set of build targets (one per registered module) and a global build target. For example, a counting target might provide a count global target to count all files, and a <MODULE>_count to count the files of a given module.
The CMake system supplies several targets that are useful for all projects and thus are included as part of the CMake system. These targets are defined in cmake/target.
See the Customization Guide for a description of adding custom targets.