What’s the difference between ‘hier(7)’ and ‘file-hierarchy(7)’ man pages?

Both hier(7) and file-hierarchy(7) man pages claim to describe the conventional file system hierarchy. However, there are some differences between them. For example, hier(7) describes /opt and /var/crash, but file-hierarchy(7) does not. What are the differences between these two descriptions. Which one do real Linux systems use?