  We study the classes of modules which are generated by a silting module. In
the case of either hereditary or perfect rings it is proved that these are
exactly the torsion $\mathcal{T}$ such that the regular module has a special
$\mathcal{T}$-preenvelope. In particular every torsion enveloping class in
$\textrm{Mod-} R$ are of the form $\mathrm{Gen}(T)$ for a minimal silting
module $T$. For the dual case we obtain for general rings that the covering
torsion-free classes of modules are exactly the classes of the form
$\mathrm{Cogen}(T)$, where $T$ is a cosilting module.
