how to judge a model is what kind of model??

I try to build a RTD model. I use the network analyzer to test the device's s-parameters. and then use the equivalent circuit to extract the components' values(R C L). a professor asked me what kind of model i am trying to build? anyone can give me a clue how to know your model belongs to what kind?
I met a lot of devices' models. They said it is a small signal, large signal, high frequency, noise, temperature, unified, physical............ How do they classify them. By the purpose of the model will be used in the future? or by the way the model is used?????