The thermal overload used in many fixtures is comprised of a Bi-Metallic spring contact. The metallic contact will bend after a certain level of heat has been put into it - either from ambient temperature or load current through it, or both.
Simply stated, when the fixture gets too hot, or there is too much load current flowing through the fixture, or both - the thermal cutout will open and stop the current flow. Once things cool down to a certain level, the contacts spring back closed, which once again allows current to flow. If the temperature problem remains, the entire cycle will repeat again and again [and again and again and again - etc.].
The consept for it is to reduce the risk of fire, mainly the result of using a lamp of higher wattage than the fixture is designed for [exceeding it's rating].
With current type cutouts, they react to overloads - similar to a fuse, but this fuse doesn't need to be replaced. These cutouts can only trip a few times before they begin to fail. Failure can be either tripping at real low levels [5 amps instead of 15 amps], blowing open/non-resetable [fried contact andlos of spring tension, plus barbecued latching points], and lastly fried closed [major fire hazard potential]. These cannot handle overcurrents too well, so that's what differs them majorly with how a fuse works. Other than the fact that the fuse elements blow out - requiring replacement - they are very similar. I am referring to small glass body fuses [the little "automotive" types and such], rather than anything like an RFN or RK fuse.
Not sure whom manufactures which device per company, so best bet is to try asking through E-Mail comments at several lighting fixture companies' web sites.
If you can ID a maker on any thermals, try using that company's name with a search engine.
Lastly, ask your supplier if they have any hints.
The thermals used in Class P ballasts are, in a way, similar in construction to those used in regular Incandescent/Halogen fixtures - along with the "Circuit Breaker" found on plug strips and even thermals on small electric motors. The Class P ones are inside the "F Can" of a ballast.
Let us know if you have specific info needed on this.
Scott " 35 " Thompson Just Say NO To Green Eggs And Ham!