From ADempiere ERP Wiki
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.