I understand that fields in a PDF form can be organized hierachically and that a "Child" field inherits some properties of the "Parent" field. How is this parent-child relationship (between fields) established using Adobe Acrobat Professional (6.0)?

As an experiment, I tried using . (dot) in the field name under the General tab of the properties dialog as follows: I started with a blank PDF and created a text field with the name "a". Next I tried to create a text field with the name "a.b", so that first text field ("a") would be the parent and this one would become a child with the name "b". However, I got this error: "An existing field of a different type already has this name......" eventhough these were the only fields in the PDF.

Am I doing something wrong or is creating field hierarchies with Acrobat Professional 6.0 not possible at all?