Your JetBlack VOLT™ EMS Smart Trainer comes with a pre-installed 11-speed cassette. You can attach other cassettes. Watch this short video for details.
Before attaching a 9-speed or 10-speed cassette, you need to put the 1.8mm cassette washer onto the cassette body first and then secure the cassette tightly on the VOLT™ V2 cassette body as per the instructions in the video guide.