Heating cartridge characteristic curve?
The energy you put in will be converted to thermal energy, distributed among the thermal mass. No energy is lost.
So, for a given energy input, you will get a certain temperature difference compared to before, which will then be transported away depending on the thermal conductivity of the interfaces between the actual heating element and anything connected to it.
This, again, is a linear process, depending on temperature difference. The cartridge is likely designed to have a hot side with good thermal conductivity, and the other sides are meant to stay cool.
Your limits are given by the maximum power rating of the components and the maximum temperature -- these are the points where you're supposed to stop feeding more energy in.
If thermal conductivity of what you've connected is bad, you can hit the maximum temperature, but stay well below the maximum power -- the temperature just keeps increasing, but can't go anywhere.
If thermal conductivity of what you've connected is good, you can hit the maximum power but stay below the maximum temperature -- any heat you generate is carried away instantly.
Somewhere in the middle between that is where you want to be, that's not defined by the cartridge however, but by whatever you connect to it.