At the product query details page you can create a new product query or edit an existing query.
The product query configuration section:

At the product query configuration section you can select a query title and a product field group you want to use for your product query.
After you select a product field group (product query category), the product query item list will be populated with all the product fields of the selected product field group.
Please note: Since the product query depends on the selected product field group, selecting a product query category will remove all previously added product fields from the product query.
The product query items section:

At the product query configuration section you can edit which product fields of the selected product field group (product query category) you want to include in your query.
The select box at the left side list all currently available product fields that were not added to this query yet.
To add a product field to a query, select a product field on the left select box (available items) and click on the arrow pointing to the select box to the right side (selected items).
The selected item will be added to the selected items list.
To remove a product field from the query, select a product field from the selected items list and click on the arrow pointing to the available items list. The product field will be removed and readded to the list with the available items.
You can also doubleclick with your mouse on any item on the list to either add or remove it from the product query.
Once you added all desired items to your product query, click on the “Save Changes” button to save this query.