CLASSPATH vs. MODULEPATH software can be run with either a CLASSPATH or a MODULEPATH.
With a CLASSPATH

The simplest way to run is via the CLASSPATH. That is because the CLASSPATH is used by the scripts which start configurations and by the handy java -jar derbyrun.jar shorthand for booting those configurations.
With a MODULEPATH

The MODULEPATH should be used if you want the extra security of module-level encapsulation which Java 9 introduced. In addition, the MODULEPATH should be used to reduce static footprint when assembling shrink-wrapped applications via the JDK's jlink tool.