  We propose a theoretical explanation of absorption/emission line systems in
classical novae based on a fully self-consistent nova explosion model. We found
that a reverse shock is formed far outside the photosphere ($\gtrsim 10^{13}$
cm) because later-ejected mass with a faster velocity collides with
earlier-ejected matter. Optically thick winds blow continuously at a rate of
$\sim 10^{-4} ~M_\odot$ yr$^{-1}$ near the optical maximum, but its velocity
decreases toward the optical maximum and increases afterward, so that the shock
arises only after the optical maximum. The nova ejecta is divided by the shock
into three parts, the outermost expanding gas (earliest wind before maximum),
shocked shell, and inner fast wind, which respectively contribute to
pre-maximum, principal, and diffuse-enhanced absorption/emission line systems.
A large part of nova ejecta is eventually confined to the shocked shell. The
appearance of the principal system is consistent with the emergence of a shock.
This shock is strong enough to explain thermal hard X-ray emissions. The
shocked layer has a high temperature of $k T_{\rm sh} \sim 1$ keV $\times
((v_{\rm wind} - v_{\rm shock})/{\rm 1000 ~km~s}^{-1})^2 ={\rm 1 ~keV}\times
((v_{\rm d} - v_{\rm p})/{\rm 1000 ~km~s}^{-1})^2$, where $v_{\rm d} - v_{\rm
p}$ is the velocity difference between the diffuse-enhanced ($v_{\rm d}$) and
principal ($v_{\rm p}$) systems. We compare a 1.3 $M_\odot$ white dwarf model
with the observational properties of the GeV gamma-ray detected classical nova
V5856 Sgr (ASASSN-16ma) and discuss what kind of novae can produce GeV
gamma-ray emissions.
