It is my impression that a cassette toilet is sort of a cross between a fixed, black tank setup and a Porta-Potti.
Like a PP it consists of two parts, the seat and the storage tank. However, the storage tank slides out of the side of the rig like a drawer. I think it is somewhat bigger than the storage tanks on the small PPs one needs to use in these rigs. Once you slide the drawer out, then you have essentially a Porta-Potti tank which you empty the same way. See attached picture from
Lil Snoozy website. This link gives a good explanation:
Thetford Cassette Toilet Systems | Truck Camper Magazine
NB: I don't know about black water tanks, but CTs seem to have a sealed black tank like a Porta-Potti. If so, always open the flush valve for the first time at a campsite with the bowl empty and the lid closed. If you ascend a mountain the air pressure in the tank will be at the lower altitude, i.e. higher, and you may get sprayed when flushing. Definitely not a pleasant prospect with debris in the bowl.
__________________
A charter member of the Buffalo Plaid Brigade!
Whether you think you can or think you can't, you're right.