A rule is a special kind of microflow. Its result should be an enumeration or a boolean and it can be used in an exclusive split to make a decision based on that result. The idea is that complicated decisions can be consolidated in rules and reused in various places.
Rules are very similar to microflows; see the documentation on Microflows for more information on how to build a rule. There are only a few differences between rules and microflows:
A rule can only be used in an exclusive split
The return type has to be Boolean or enumeration
A rule has a different background in the Modeler so that you can quickly spot whether you are working on a rule
A rule cannot change data in the database; the actions to create, delete, change and rollback objects are not available in rules
A rule cannot interact with the client; the actions to show or close forms, show messages, send validation feedback and download files are not available in rules
A rule cannot call web services, generate documents or import XML