Low Frequency RFID uses magnetic fields from an antenna to wirelessly power a PIT tag so it can transmit an identification number. The tag has no internal power source and only operates in the presence of a reader. The oscillating field produces a small voltage via magnetic induction through a coil of wire that is inside the tag.
Magnetic signals can travel through most non-metallic materials, including water, wood, plastic, glass, concrete, and dirt. HF and UHF radio signals do not conduct well through water so low frequency RFID is the ideal choice for fish and wildlife tracking.
The ISO 11784/11785 standard specifies protocols so that tags and readers from different manufacturers will work together. FDX and HDX are interoperable but not compatible. Like AM and FM radios, there are PIT tag readers that detect FDX only, HDX only or both.
The two technologies have different strengths and capabilities so the choice depends on the how it will be used.