The installer sets this variable to the full path of the native version of Program Files folder on the target system. On 64-bit Windows versions, this variable refers to the 64-bit Program Files root folder; on 32-bit Windows versions, it's the 32-bit Program Files root folder.
This path should be used for programs that are neither 32-bit nor 64-bit specific, for example MSIL-based .Net applications.
You can refer to this variable as <ProgramFilesXFolder>. Do not change its value; it has been carefully configured for cross-platform compatibility.