How many diffs do you need to do to get a stationary process?
If something is first order integrated it is \(I(1)\).
If we can remove the trend as a function, eg linear or non-linear growth, and the rest is stationary, then the process is trend stationary
We can model the process as:
\(y_t=\mu_t +f(t)+\epsilon_t \)
We can have shocks having effects over time.
This is separate to trends.