If the installer is running on an x64-architecture processor (AMD-64 or EM64T), the installer sets this variable to the processor level as defined by the Win32 SYSTEM_INFO::wProcessorLevel data structure. If the installer is running on another processor, this variable is not set.
You can refer to it as <Msix64>. In conditional expressions you can omit the brackets and use tests like Msix64 to test for the presence of an x64 processor. Do not set this variable directly; it is set automatically when the installer runs.