A combination of using top level categories and sub level categories and combining with dynamic placeholders gives us a very powerful search query engine. Each site will have different search requirements and be setup differently. We assign top level and sub categories to categorize our search. So we could have interests as a top level category then sub categories all the interests. We can add an unlimited number of top level and sub level categories. Categories can be rendered with a counter to show the number of items pertaining to this category. Categories can be rendered in different formats such as drop down, radio box, check box and basic links.

To generate drop down lists for selection and use on the site
We need to extract the sub level categories as a drop down list - this can then made by as a selection by the web user. We can have multiple drop down lists. Lists can also be used in combination with other search queries such as search by type, keyword.
If we add the placeholder submit button the lists will act as an AND search query so a very targeted search query can be made if no search button has been added each drop down will auto submit so search will be made on the one selected category only. Both systems are equally popular.

An example of a search query placeholder targeting a set of sub categories.

Brand

So our top level category is brand and the ID of the top level category we need to enter is [0000010653] as displayed to the right of the category.
We have 3 category lists so this is labelled form-widget-1, our other lists are labelled ....form-widget-2     ....form-widget-3 etc....

The [L] specifies the type of render such as
[L] Text links
[D] Drop down list
[R] Radio Box
[C] Check box

If we are running  a directory we will have 2 category systems categories for the products and categories for the profiles.
We need to supply if we are targeting categories or profiles we do this by supplying:
$form-widget-1[LIST][L][00000XXXXX]$ will target listing categories
$form-widget-1[PROF][L][00000XXXXX]$ will target profile cateories

Click here to see a detailed description of each form placeholder

For more information and help on this
Please see managing placeholders.
Please see detailed form placeholders