/usr/lib includes object files, libraries, and internal binaries that are not intended to be executed directly by users or shell scripts.
Applications may use a single subdirectory under /usr/lib. If an application uses a subdirectory, all architecture-dependent data exclusively used by the application should be placed within that subdirectory. For example, the perl5 subdirectory for Perl 5 modules and libraries.
Miscellaneous architecture-independent application-specific static files and subdirectories should be placed in /usr/share.
Some executable commands such as makewhatis and sendmail have also been traditionally placed in /usr/lib. makewhatis is an internal binary and should be placed in a binary directory; users access only catman. Newer sendmail binaries are now placed by default in /usr/sbin; a symbolic link should remain from /usr/lib. Additionally, systems using Smail should place Smail in /usr/sbin/smail, and /usr/sbin/sendmail should be a symbolic link to it.
A symbolic link /usr/lib/X11 pointing to the lib/X11 directory of the default X distribution is required if X is installed.
Note: No host-specific data for the X Window System should be stored in /usr/lib/X11. Host-specific configuration files such as Xconfig or XF86Config should be stored in /etc/X11. This should include 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).