additive nonlinear system

Question

Could anyone tell me what does it mean by additive non-linear system? It would be fine if someone help me with an example. for example

$\dot x=f(x,u)=Ax+E\phi(u,x)+Bu, y=Cx$ is additive non-linear.

I know $\dot x= Ax+Bu, y=Cx$ is linear system.