  Because mass loss is a fundamental phenomenon in massive stars, interaction
with circumstellar material (CSM) should be universal in core-collapse
supernovae (SNe). Leaving aside the extreme CSM density, extent, or mass
typically encountered in Type IIn SNe, we investigate the diverse long-term
radiative signatures of interaction between a Type II SN ejecta and CSM
corresponding to mass loss rates up to 10$^{-3}$ $M_{\odot}$ yr$^{-1}$. Because
these CSM are relatively tenuous and optically-thin to electron-scattering
beyond a few stellar radii, radiation hydrodynamics is not essential and one
may treat the interaction directly as an additional power source in the
non-local thermodynamic equilibrium radiative transfer problem. The CSM
accumulated since shock breakout forms a dense shell in the outer ejecta and
leads to high-velocity absorption features in spectral lines, even for
negligible shock power. Besides Balmer lines, such features may appear in NaID,
HeI lines etc. A stronger interaction strengthens the continuum flux
(preferentially in the UV), quenches the absorption of P-Cygni profiles, boosts
the MgII $\lambda\lambda$ $2795,2802$ doublet, and fosters the production of a
broad boxy H$\alpha$ emission component. The rise in ionization in the outer
ejecta may quench some lines (e.g., the CaII near-infrared triplet). The
interaction power emerges preferentially in the UV, in particular at later
times, shifting the optical color to the blue, but increasing modestly the
optical luminosity. Strong thermalization and clumping seem to be required to
make an interaction superluminous in the optical. The UV range contains
essential signatures that provide critical constraints to infer the mass loss
history and inner workings of core-collapse SN progenitors at death.
