Applications may use a single subdirectory under /usr/lib. If an application uses a subdirectory, all architecture-dependent data exclusively used by the application must be placed within that subdirectory.[footnote 19]
For historical reasons, /usr/lib/sendmail must be a symbolic link to /usr/sbin/sendmail if the latter exists.[footnote 20]
If /lib/X11 exists, /usr/lib/X11 must be a symbolic link to /lib/X11, or to whatever /lib/X11 is a symbolic link to.[footnote 21]
[18] Miscellaneous architecture-independent application-specific static files and subdirectories must be placed in /usr/share.
[19] For example, the perl5 subdirectory for Perl 5 modules and libraries.
[20] Some executable commands such as makewhatis and sendmail have also been traditionally placed in /usr/lib. makewhatis is an internal binary and must be placed in a binary directory; users access only catman. Newer sendmail binaries are now placed by default in /usr/sbin. Additionally, systems using a sendmail-compatible mail transfer agent must provide /usr/sbin/sendmail as a symbolic link to the appropriate executable.
[21] Host-specific data for the X Window System must not be stored in /usr/lib/X11. Host-specific configuration files such as Xconfig or XF86Config must be stored in /etc/X11. This includes configuration data such as system.twmrc even if it is only made a symbolic link to a more global configuration file (probably in /usr/X11R6/lib/X11).