How much can a cargo plane take?

I played many years before, but I decided to try running a cargo airline which I never did. I got some DC-9-40F HK ones, and assumed that on short tracks, it will book either to max. cargo capacity (19,637 kg) , or max payload (14,360 kg). However, even on the short SDF to ORD flight, it only books 138 units. What are this units, and how are these calculated? Which params should I look for on a cargo plane to estimate the real bookable capacity?

Hey, sorry for the late reply. I wonder why nobody answered it, as it’s really simple: 1 Cargo Unit (CU) = 100kg :slight_smile: