causality - Causal system and Physical Systems

According to the Paley-Wiener criterion, a system is causal if satisfies:

$$\int\limits_{-\infty }^{+\infty }{\frac{\ln (|H(f)|)}{1+{{f}^{2}}}}df<\infty$$

So I want to know

1. This equation is related to LTI system only?


2. Only casual system is realizable?for linear and non-linear systems?


3. Which kinds of systems can we create?(only Paley-Wiener criterion can be enough?)