In DJ-Classifieds you can decide if you want to allow your users to find the items with predefined filters for custom fields or also give them possibility to search by all custom fields' results.
Once adding a new custom field you provide the values.
If you want the empty selection be available in default in filter/search module you need to add ';' sign at start.
There are two scenarios available:
1) - you give the search all (empty) select option - so once that is selected by user on frontend it searches all the values available in this custom field
2) - you just specify the values without providing the empty value - so user that uses the search/filter module on frontend can just pick the predefined values.