I came across the Brunauer-Emmet-Teller (BET) equation recently, and one thing that I found strange was an implicit reference to the formation of molecular multi-layer.
In the BET equation
$$ \begin{align} \frac{P}{V(P^0 - P)} &= \frac{1}{CV_m} + \frac{C-1}{CV_m}\cdot \frac{P}{P^0}\, ; \tag{1}\\ C &= e^{(Q_1 - Q_2)/RT}\quad (C:\text{constant})\, ,\tag{2} \end{align} $$
where $V_m$ – monolayer adsorption amount, V – adsorption amount at the equilibrium pressure $P$, $Q_\mathrm{L}$ refers to the heat of adsorption of the second and higher layers of adsorbate onto the adsorbent.
Now according to my textbook, chemisorption only results in the formation of a molecular monolayer. Multilayers are formed by physisorption.
The thing is, I'm told the BET equation is an extension of Langmuir's adsorption isotherm for chemisorption at low pressures. If the BET equation applies only to chemisorption, then what is the point of including a term dependent on the formation of multilayers, when chemisorption doesn't even form a multilayer all by itself.
Is there something I'm overlooking here? The internet has not yielded any decent result in that regard.