I think by cassette he is referring to the name the popup mfrs (I like MFers better than manufacturers--I think it captures the truth better) give to the entire shower/toilet enclosure. So the OP needs to determine whether he has a true porta-potti (removable from the camper) or a regular RV toilet, with fresh water supply and connection to a waste holding tank.
The Mfers refer to it as a cassette.
It is a porta potti that slides out of the pup.
When I slid it out I found it covered with water.
Dont know why it was covered in water but if anyone
has any theories, please chime in.
We had this Thetford Potti in our '95 Jayco.
When you swivel out the water fill (gray plastic above the Thetford sticker in your pic) and begin filling with water, that clear tube displays the water level. We used to fill to somewhere about even with the Thetford sticker. It makes it easy to see how much "flushing" water you have available.
Further, if you live where you have to drain the water out, the tube pops out and you point it down to drain.
Our potti was in the shower area. We used to occasionally get a bit of water on the removable tank when using the shower. Nothing like that though!!
It was a good system, easy to dump in any toilet or at the dump station.
PS-the toilet chemical was poured into the big gray "cassette" through the top hole, not in with the fresh flush water.
PSS-final tip....when pouring, "dumping", out the waste I remember a yellow button (not seen in your pic) that was pushed while tipping tank up to let air in and make a smooth, splashless, dump.