Why does the Pilatus PC-24 have such a large "Airplane Support"?
To add to the other answer...

The Pilatus PC-24 is billed as a business jet that can operate out of rough airfields. Comparing the pictures of the PC-24 to the Phenom 300 or CJ4, you can see that there is quite a bit of extra ground clearance. You can also see that the rear landing gear on the PC-24 are much more robust (dual wheels, longer travel length) than the other two aircraft. 

If you look at this picture:


Source: Wikipedia

You can see that the wheels also retract into that area. They probably wanted to keep the wing low to take advantage of ground-effect to improve short-field performance. So having a higher-up fuselage allowed them to put the engines up higher and avoid FOD damage, while the low wing allowed them an impressive short field take-off performance. The PC-24 can operate out of a field just 2,800 feet long. The Phenom 300 takes 3,200 feet and the CJ4 takes about 3,500.

So the "bump" is there to be able to handle additional loads, elevate the fuselage, and store the gear. I can't find any diagrams of the fuel system, but it's possible that it also houses fuel tanks or baggage.