Used by WixInternalUIBootstrapperApplication to determine which package is the primary package. The default value is "default". This package is used unless there is a package for the runtime machine's architecture. This package is used when the runtime machine's OS is x86 (IMAGE_FILE_MACHINE_I386). This package is used when the runtime machine's OS is x64 (IMAGE_FILE_MACHINE_AMD64). This package is used when the runtime machine's OS is ARM64 (IMAGE_FILE_MACHINE_ARM64).