When not cross-compiling, this variable has the same value as the CMAKE_HOST_SYSTEM_PROCESSOR variable. In many cases, this will correspond to the target architecture for the build, but this is not guaranteed. (E.g. on Windows, the host may be AMD64 even when using a MSVC cl compiler with a 32-bit target.)

When cross-compiling, a CMAKE_TOOLCHAIN_FILE should set the CMAKE_SYSTEM_PROCESSOR variable to match target architecture that it specifies (via CMAKE_<LANG>_COMPILER and perhaps CMAKE_<LANG>_COMPILER_TARGET).