What does it mean by model is fully linear?

Hi,

I am solving a problem where I am designing a metamaterial using topology optimization (in Optistruct) to mimic the deformation behavior of a traditional elastomer. For this, there are three prescribed loading cases under which the metamaterial must undergo a given value of deformation; for example, for 20000N load, the deformation should be 5mm. I have three load steps for the three loading cases. I am using non-linear quasi static analysis. However, when I generate the result, the .out file says the model is fully linear. What does it mean by 'model is fully linear' ? Attached is the .out file.