I can't find a definitive answer to why Ubuntu would set the CATALINA_BASE environment variable, making it different from CATALINA_HOME. I found this document which suggests that not setting CATALINA_BASE would make it equivalent to CATALINA_HOME. That document also suggests that setting CATALINA_BASE would be done to make multiple instances available.
However, I don't seem to be able to find anything that explains this. Presumably, running with a single CATALINA_HOME has been good for a long time, but Ubuntu switched.
I found this question with an illuminating answer on AskUbuntu but there isn't much specifically on CATALINA_HOME vs. CATALINA_BASE. That question does allude to the "appBase" parameter, which is documented in the Tomcat6 documentation for the Host container in the configuration file.
Here's the questions: Why are these two variables separated? When should they be separated? What does this allow the administrator to do? What do you then place into each directory? When should these two variables be the same? What does ASF recommend for standard practice?