If you know some or all of the product number, you may enter that in. The same is for the description. If you know the entire description or just a word, you may enter that in. In either case, be sure Product Category is set to No category.
For example: Let's say you want to see what kinds of sheet rock we offer.
Product Category would be set to No category.
Product Number would be left blank.
Product Description would be set to "rock"
The click the submit button.