Hi @ceveleigh I'll try and explain in other words, using your example:
In the case you describe, the 1100°C and 100°C correspond to the Initial Conditions for Temperature.
On the other hand, the reference temperature is the value at which the thermal deformation (and hence the stress) of a material is zero. If the material temperature deviates from this value, compression or extension will occur and stress will be developed.
To answer your question, to define the reference temperature for a material, you look at the thermal expansion model, it should give you two parameters: the expansion coefficient and the reference temperature (at which the expansion is zero). So you should have those two parameters for both of your materials. If a reference temperature is not given, it is often assumed to be ambient temperature (25°C).