Alternative File Descriptions appear in many places within TestBench and are a means of describing additional new data layouts to the parts of the product that would normally be limited to the IBM i OS/400 provided file description. Additional fields can be defined that are made up of sections of existing fields or which are a combination of fields. This means that a file layout can be manipulated before it is used providing a very powerful extension to the facilities of TestBench.

There are two principal functions within AFD:-

1. Substring – Create a new field from part of an existing field.
2. Concatenation – Create a new field by joining together existing or sub-string fields.