Near Field Electrotherapy: The patient is placed within the electromagnetic field generated by the device. The electric field interacts directly with the tissues, and energy transfer occurs by conduction and displacement currents through the body. Examples include shortwave diathermy (capacitive and inductive methods) and microwave diathermy at close range.

Far Field Electrotherapy: The patient is placed at a distance from the source, in the radiated electromagnetic field (the "radiation zone"). Energy is transferred by propagating electromagnetic waves that travel through space before being absorbed by tissues. The field behaves as a plane wave. Examples include microwave diathermy where the applicator is held some distance from the skin.

Here is a clean 3-mark answer on Near Field and Far Field Electrotherapy:

In near field electrotherapy, the patient's tissues are placed within the electromagnetic field generated by the applicator (i.e., the treatment distance is much less than one wavelength of the radiation used). The electromagnetic field in this region is non-propagating and dominated by inductive and capacitive (reactive) effects. The electric and magnetic fields behave independently and do not form true travelling waves. Energy is transferred directly to the tissues by conduction currents and displacement currents rather than by radiated waves. The field strength decreases rapidly with distance. Short wave diathermy (SWD) - both the capacitor plate (condenser field) and inductive coil (inductothermy) methods - operates in the near field.

In far field electrotherapy, the patient is positioned at a distance much greater than the wavelength from the applicator (r >> 2λ), placing them in the true radiation zone. In this region, the electromagnetic energy propagates outward as coupled, plane-like travelling waves (electric and magnetic fields are in phase and perpendicular to each other). Field strength decreases as an inverse square of distance. Energy is absorbed by tissues as the propagating wave enters them. Microwave diathermy (MWD) is the classic example, where the director/applicator is held several centimetres from the skin surface and the microwave beam travels through space before being absorbed.