The CATEGORY field is the way FarApp determines which set of mappings to use for a product. The vast majority of storefronts use only a single mapping category. In cases like those we use a special mapping rule on the backend in combination with the flag field that essentially looks to see if the flag field has a non-empty value. If it does, then we know to use the particular mapping CATEGORY in question. The logic is essentially "Do we sync this item to this storefront? If we do then use this mapping category" because there is only a single mapping category in use.
When this logic is in use, the UI will display the field ID of the flag field in the "NetSuite Field ID" field and a blank "Value" field as seen below:
Note that this is not the ONLY case where those fields will contain that combination of values but it is by far the most common configuration when this previously described special mapping is in use.
In short, the only time you need be concerned about a blank value field is if your items are not syncing and displaying a message in the FarApp dashboard about not being able to categorize.