Rules are business rules coded in scripts. Different types of scripting are are supported. You can create rules to be executed for some event occurs such as a field in the window changes (Callouts), a table record change ModelValidator Table Event, user logge in event ModelValidator Login Event, a document is processed ModelValidator Table Event or a process is triggered.
You can create the rule from ADempiere menu: Application Dictionary >> Rule. Once you create the rule, you can use this rule when you define a Callout, Model Validator or Process.
Script editing tool will help you to write desired script.