Rules play a crucial role in Transaction objects, as they allow you to program their behavior (for example: assigning default values, defining controls over the data, etc.).
They are written in a declarative way, which means the order in which they are written is not necessarily the order in which they will be executed.
All Transaction rules can involve attributes of the Base Tables associated with the Transaction and most rules can also involve attributes of the Extended Tables of those base tables, but they must be included in the Transaction's structure.
In addition, many Transaction rules allow involving variables defined within the object, constants, and functions.
The rules defined in Transactions are local. That is, they're only evaluated and executed (if applicable) when executing the Transaction in which they are defined (the Transaction UI or as a Business Component).
Some valid rules for Transactions are:
(*) These are conditional rules and you can define triggering conditions for them.
Transaction Rules Syntax
Triggering context for Events and Rules
Rules definitions