It is stated in the LLOB model (i.e. this paper, https://arxiv.org/pdf/1105.1694.pdf ) :
Our basic idea is that of a “latent order book” that at any instant of time t aggregates the total intended volume for sells at price p or above, $V+(p, t)$, and the total intended volume for buys at price $p$ or below, $V−(p, t)$. We want to emphasize that this is in general not the volume revealed in the real (observable) order book, in particular for p remote from the current price $p_t$ . It is rather the volume that would reveal itself in the order book, or as market orders, if the price came instantaneously closer to $p$. But since there is little incentive to reveal one’s intentions too early, most of the volume is latent and not revealed.
Can anyone explain why there is a certain volume that is not displayed?
Also, why do we have the condition $\rho_A(p_A,t)=\rho_B(p_B,t)$ if we denote $\rho_A$ and $\rho_B$ the ask and bid densities ?