Self-regulating heat tracing tapes are cables whose resistance varies with temperature - low resistance for temperatures below the cable set point and high resistance for temperatures above the cable set point. When the cable temperature reaches the set point, the resistance reaches a high point, resulting in no more heat being supplied.
These cables use two parallel bus wires which carry electricity but do not create significant heat. They are encased in a semi-conductive polymer. This polymer is loaded with carbon; as the polymer element heats, it allows less current to flow so the cable is inherently power saving and only delivering heat and power where and when required by the system.  The cables are manufactured and then irradiated and by varying both the carbon content and the dosage then different tape with different output characteristics can be produced. The benefits of this cable are the ability to cut to length in the field. It is more rugged, and much more reliable than a constant wattage cable; it cannot over-heat itself so it can be crossed over, but it is bad practice to install tape in this way.  Self-regulating and constant wattage heating cables have specific maximum exposure temperature, which means that if they are subject to high temperatures then the tape can be damaged beyond repair.
Also self-limiting tapes are subject to higher inrush currents on cold starting up similar to an induction motor, so a higher rated contactor is required.