Qu'est-ce que « State-space model, local level model » ?

A linear Gaussian state-space model has an observation equation $y_t = Z_t\alpha_t + \varepsilon_t$, $\varepsilon_t \sim \mathcal N(0, H)$, and a state equation $\alpha_{t+1} = T\alpha_t + \eta_t$, $\eta_t \sim \mathcal N(0, Q)$, with independent noises and $\alpha_1 \sim \mathcal N(a_1, P_1)$. The local level model is the scalar case $Z = T = 1$: an observed series equals a random-walk level plus noise.