The upper-frequency limit for Contact Mode is limited to the frequency generator being able to sustain a high enough output voltage to overcome skin/tissue/TENS pad resistance to provide sufficient current to provide an effective treatment. The higher you go up in the MHz frequency range, the more cable length, wire/cable type characteristics, and impedance matching affect signal quality.
XM generator has a 5MHz frequency limit. Higher frequencies can be reached using Spooky2 wave cycle multiplier function.
GX/Pro generator has a 40MHz upper frequency limit. Higher frequencies can be reached using Spooky2 wave cycle multiplier function.
For more details, please check the link: