Max temperature of PTC heater, need tip
Any particular PTC device will have a resistance value that depends on its absolute temperature. Given this information, plus the voltage at which you're going to operate it, you can then compute how much power it will absorb as a function of temperature.

PIN = V2/R(T)

Since PTC means that the resistance rises with temperature, the power curve will fall with temperature. Note that if the resistance rises sharply at a particular temperature, the power curve will fall sharply at that temperature.

The geometry and materials of the PTC and its connection to the application will determine how much heat flows out of the device at any particular temperature. This will depend on things such as whether the heat flow is conducive, convective, or radiative.

POUT = f(T)

This will generally be a rising polynomial curve, with the exact exponent depending on the dominant cooling mechanism.

You can then derive the operating temperature of the device by drawing both of these curves on the same chart and seeing where they cross. If the PIN curve falls off sharply, then the temperature of the crossover will have less of a dependency on the exact cooling mechanism.

There is a huge amount of flexibility in manufacturing thick-film PTC devices. You can choose among a number of film formulations, each of which will have a particular resistivity (ohms per square) vs. temperature curve. You then have a wide range of options in terms of the pattern you use to print the film &mdash; you can make it long and narrow (even serpentine) if you want high overall resistance, or short and wide if you want low resistance. And of course, the actual operating voltage is another variable.

With all of these variables, and the huge number of "standard parts" that the vendors offer, it would be impossible to produce a comprehensive datasheet. This is why you need to talk to their application engineers, at least until you have a better intuitive feel for how these things operate.