In order to query by category field, your documents must have such fields set. This is usually done by mappings. For example, a Denote-related mapping might extract keywords and add those as categories. You can then proceed to add a category query by pressing P (p-search-add-prior) then c.
You will be prompted to select a category field that your document has (e.g. “keywords”), then from all of the available values you’ll select the one you want to query for. Finish by creating your prior and then the documents will be re-scored.