The installer sets this variable to the full path of the 64-bit Drivers folder on the target system. If the target system is 32-bit, then the folder path may refer to a non-existing folder. However, if you install files into the folder, it will be automatically created (even though the target system will probably ignore its contents).
You can refer to this variable as <Drivers64Folder>. Do not change its value; it has been carefully configured for cross-platform compatibility.