Currently 'Common' classes and libraries are searched from $CATALINA_HOME, where they should better be located in $CATALINA_BASE. At the very least, $CATALINA_BASE/common/lib and $CATALINA_BASE/common/classes should be supplementary. Consider a shared hosting environment where the Tomcat core classes are kept read-only by a super-user, and entities run (many) instances of Tomcat from their own $CATALINA_BASE directories. Putting common jars in $CATALINA_HOME/common/lib isn't an option if two entities are using incompatible versions of the same library. Appending $CATALINA_BASE/common/lib and classes should break very, very few existing installations.
"common" is considered to be the server binary, the same way "server" is. You have the "lib" and "classes" folder for what you want to do.
Very well, the 'shared' category, then... ...but both "lib" and "classes" are resolved relative to CATALINA_HOME, where they should more properly resolved to CATALINA_BASE.
*** This bug has been marked as a duplicate of bug 10060 ***