Classified Ads are sorted into Categories (and then further sorted into Sub-Categories) in order to group similar ads together. This lets people easily browse through ads that they are interested in. When creating a Classified ad, you must pick a category. After you have selected a main category, the sub-category list will be updated to contain all of the sub-categories for the category you selected; a sub-category must be selected.
Both categories and sub-categories have a price. After you have selected a category and sub-category, the total cost to post into your selected categories will be displayed; if the selected categories are provided free of charge, you will see a message informing you that there is no cost associated with your selection. If there is a cost for either the category or sub-category, you will be shown the price for both.