When you select a category within the single search interface with the steps described below, you can either browse through the products or use the single search to search only within this category. This means that you will not find any products outside the category using this method. If you want to search for a product across all categories of the seller, you need to use the GLOBAL SEARCH option. Click here to display the help file that explains that search option.
You can use the single search interface to move through the category structure of a seller. Here's an example:
Click on the SELECT arrow to select a category:

A drop-down list will display the available categories:

Click on GO to move to the category you selected.
In this example of the Auravita site, it is the product category "Personal
Care".

The category path shows you at any time the point at which you are
within the category structure:

You can now move further down in the category
structure by selecting a subcategory and clicking on GO:

The category path will show how you have moved down in the structure:

Once you have located the category within which you want to conduct your search, you can just type the search item in the field to the right of the category selection and click on GO: