PLUGINURL is used by Netscape Communicator's
JAR Installation Manager.
PLUGINURL points to a "JAR archive" which is a resource used to automatically install the plugin (so they say, I've had all kinds of problems with Netscape's "automatic" stuff).
JAR archives are a special type of compressed files originally used for Java; see
For more details I refer you to Netscape's
Plugins Development Overview.