If the installer is running on an AMD 64-bit processor, the installer sets this variable to the AMD 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 <MsiAMD64>. In conditional expressions you can omit the brackets and use tests like MsiAMD64 to test for the presence of an AMD 64-bit processor. Do not set this variable directly; it is set automatically when the installer runs.