what is difference between shared.loader and common.loader

[Tim McGuire]

the tomcat class loader howto (http://tomcat.apache.org/tomcat-5.5-doc/class-loader-howto.html)

states that:

Shared - This class loader is the place to put classes and resources that you wish to share across ALL web applications (unless Tomcat internal classes also need access, in which case you should put them in the Common class loader instead)

They say "should" instead of "must".

in this case does "should" mean that Tomcat internal classes cannot read these UNLESS they are put in Common?

my situation is that I don't have permission to place jars in tomcat's Common directory, but I do in shared. I don't fully understand the difference between shared.loader and common.loader

can tomcat internal classes access the classes loaded by the shared.loader?

Thanks