The Jare Ruleengine comes with a lot of predefined checks. They check a certain condition like "is smaller than", "is not null", "starts with", "check matches", "list has member" etc.
And there are also several predefined actions. They are applied based on the results of the rules (or more correctly the result of a rule group). There are actions like "set value", "concat field values", "append value", "substring", "sum" etc.
Both checks and actions can be easily extended to create new ones. So if you have a situation where writing the rule logic is very cumbersome or you can't easily achieve what you need to define, then you could think of writing a check that helps you get the job done more easily.
Same for the actions. If you want or need a specifiy handling after a group of rules passes or failed the tests, then you could write an action, that does the job. E.g. log entries, sending messages, trigger another process, etc.
Both checks and actions are easy to extend, so over time you could build up a library of them, if the basic, already existing checks are not sufficient.
And it would be nice of course, if you can share your extensions with the rest of the community.