Filter....

I've been using the fish calculator to check my stocking levels, but am unsure what to class my filter as, it's external but not a canister filter, it's more like an overhead sump. The dimensions are 60cm long by 10cm by 10cm it has 8 compartments and filters 1200 liters per hour, my tank is 200lt. I've tried to attach a photo of the filter but haven't been able to.

The concept that a bigger filter means more fish is rather outdated. Provided the filter has enough media to hold all the bacteria needed by the fish and a turnover that prevents the tank becoming stagnant, it doesn't matter how big the filter is, the tank can hold the same amount of fish whether is it an internal or an oversized external.

Sumps are a different matter. They are second tank, usually placed under the main tank, and contain quite a large volume of water. This water can be added to the volume of the tank, but there must still be enough swimming room for the fish in the main tank so the tank + sump volume can't be stocked to 100%.

However, what you describe sounds more like a trickle filter built into the hood. If it is this type rather than a sump, class it as internal.