It is a kit of parts that comprises of a fan and the associated tubing, switch, wiring and either a vent housing to fit on the cassette door or a through the roof vent.
Once installed when you open the internal flap of your toilet the fan starts and causes a depression in the tank venting to the exterior of the van, thereby stopping most or all of the nasty smells associated with sitting on the toilet.
It's not an alternative to a "cassette toilet", simply an addition.
Some manufacturers have fitted them as standard in the past.
Some Thetford toilets have their own version of the idea.
For some more info see here
https://www.soguk.co.uk/