Rules play a very important 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 such base tables, but they must be included in the Transaction's structure.
Rules may involve not only the attributes included in the Transaction's structure, but also variables defined within the object, constants, and functions.
They are local, so they're only valid within the Transaction in which they are defined.
Some valid rules for transactions are:
Transaction Rules Syntax