Now to add to that, is the understanding and relationship between Categories and Listing Fields within the Flynax Script, and to explain further, is to say that for the best part a Listing Field can be seen in the same way as a Category, but with one difference.
That difference is that in most cases a Sub Category can only apply to the Parent Category of that Sub Category, while a Listing Field in most cases, applies not only to the Parent Category of that Listing Field, but also often belongs to one or more other Parent Categories.
Ask if this Sub Category belongs to one or more Parent Categories to determine if you should create a Sub Category or a Listing Field.