Custom Ad Fields are obtained through installing the Extra Fields module and configuring whatever fields you wish.  AWPCP allows you to have fields of various types (number, string, URL), as well as private/public, assign them to a particular category or make them available to all ads, and show them in the search form (or not).

To enable custom fields for your ads, do the following:

1. Purchase the Extra Fields premium module
2. Install and configure Extra Fields module
3. See your new fields in the Place Ad form, search on them in the Search Ads form.  You need to select a category FIRST before you will see category-specific fields on either page.  General fields are always visible.