Hot cathode ionization differs from cold cathode in that it has a hot filament emitting ions inbuilt into the sensor head. This technology is capable of measuring from medium to ultra-high vacuum pressures (10-12 mbar/torr), the trade off being a sensor head that requires more protection, and with its more complex design comes a higher cost of ownership.
As with cold cathode technologies, they stop measuring ~10-2 mbar/torr, therefore are normally combined with an integrated pirani to provide full range measurement in a single gauge.
Also there are different versions of the hot cathode technology, with the main types being Bayard-Alpert (ITR or IE 414) and extractor (IE 514). The extractor technology offers the lowest pressure measurement with the ability to measure to 10-12 mbar/torr