We have set our database to best practice (two-tiered) because we opine that the more sub-categories there are, the harder it will be for sellers to upload their product. It will also create a complicated (and therefore unpleasant) experience for the buyers. So in short, we have made a decision not to create more than 2 tiers.
Kindly note that this applies to Enterprise customers as well. Hence even if you are an Enterprise customer using our API to customise your site, it would be an API for only two levels which you can design and develop.