Here is a quick video on the different type of pads used in Electrical Muscle Stimulation.
This should remove some of the mysteries if you are not familiar with EMS.
I also discuss briefly on why you should not buy a cheap 2 channel EMS device and spend the extra money on getting a proper 4 channel device (regardless of brand)
This video is vendor non-specific, so it applies to any Electrical Muscle Stimulation unit.
By the way, I forgot to mention in the video that EMS replacement pads are universal, so you can always buy then at your shopping convenience. (i.e. Amazon, or any local health practitioner store or even a medical supply store)
Same goes with the gel.