jetty.base is the jetty configuration directory.
This is where the etc, webapps and start files will be looked for.
If not found in jetty.base, they are looked for in jetty.home.
A Jetty base directory allows the configuration and web applications of a server instance to be stored separately from the Jetty distribution, so that upgrades can be done with minimal disruption.
It defines the location of a specific implementation of a Jetty server, its configuration, logs and web applications (typically start.d/*.ini files, logs and webapps).
The Jetty - Getting started (Hello World) a fully-functioning Jetty Base complete with numerous web applications demonstrating different Jetty functionality.
Set as property
jetty.base=[directory]
or env variable JETTY_BASE