javaX is a package reserved for 'eXtra' classs that
are not in the Java core package.

Such things like 'mail' or 'sound' API's might be
large, and required by only a few applications,
so they are put in a 'javax' package and distributed
as a separate Jar (to the normal Java Plug-In
Run-time environment).

But.. things change over time.  Some of the 'eXtra'
packages become so regularly used that Sun decides
to include one or more of them into the core Java Plug-In.

That is where the classes in the javax package come from,
once they were all 'eXtra' things from Sun that could be
added in to applications and applets.  At Java 1.1, even
Swing was an 'eXtra' Jar.

HTH

(I wonder how long it will take a 'JLS demon' to come
along and prove how wrong, my simple explanation is?)

Andrew T.