\begin{equation}
\nu_{it} = \varepsilon_{it}
\end{equation}
You have a different structure if:
\begin{equation}
\nu_{it} = \epsilon_{it} + \theta\epsilon_{i(t-1)}
\end{equation}
\nu_{it} = \varepsilon_{it}
\end{equation}
You have a different structure if:
\begin{equation}
\nu_{it} = \epsilon_{it} + \theta\epsilon_{i(t-1)}
\end{equation}
