Automatically numbers a Transaction object’s second (or third or another nested) level.
Since the Autonumber property can only be applied to single primary keys, to automatically number a second level you can use the Serial rule.
This rule requires defining an attribute in the first level of the Transaction, which will save the last value assigned to the lines of the second level.
Serial(att1 , att2 , step);
Is the attribute to be increased.
Marks the starting value for att1, and is always updated with the last value of att2.
att2 must be included in the structure level that is immediately above that the attribute you wish to autonumber (you can delete att2 from the form, but the attribute att1 is required to be on the web form, otherwise the rule will not work on the client-side).
Is the value to add to each att1. In other words, is the serialization step or increment.