Variants can be added to categories on your auction site to enhance/enforce information related to items in that category.
For example, if you had the category "cars", you could add a variant called "kilometres travelled".
The distance travelled will then appear on the sell an item page, the public item page, and will be a searchable option on the category page itself.
Therefore a visitor could search for cars having travelled less than 100,000 kilometres.
This system can be utilised to great effect on virtually any auction site.