The hand is definitely the oldest percussion instrument. During the "disco" era in the seventies, the clap sound became very popular on many recordings made at that time, but thee claps where "handmade". How could bands bring this clap sound on stage? Dave Simmons detected this gap in the market and developed his first generation of clap synth: the analog Musicaid Claptrap. He went from studio to studio and demonstrated his invention. Basically this first version is a noise generator which can be roughly brought in the direction of a natural ensemble clap sound making use of a few parameters. This sound was popular for example for snare overdubs. The whole Claptrap series was probably one of the most successful product. Additionally the Claptrap sound is supposed to be one of the mostly swiped sounds ever
All in all an inexpensive way to get Simmons analog sounds. But onthe strength of the fact, that this was composed of pretty cheap electronic components, you should not expect the sound of an SDSV or SDS7.
As all desktop devices, the SDS 800 cannot be extended with additional modules.
The SDS 800 has been shipped with 4 Mark IV pads (structurally identical with SDS 9 and SDS 1000 pads) comprising 1 bassdrum pad and 3 small mono pads.