A family of Microsoft word processing software products for creating web, email, and print documents.
The AUTONUM field can't be customized. Note that it would be easier to use paragraph numbering as discussed in the article at http://www.shaunakelly.com/word/numbering/numbering20072010.html.
If you really need a field, you can use the LISTNUM field. You can modify its numbering if you select the field and then click Home | Multilevel List | Define New Multilevel List.