$S \subseteq T$ means $S$ is a subset of $T$.
Note that this symbol allows the possibility that $S = T$.
The $\LaTeX$ code for \(\subseteq\) is
$\subset$ is sometimes used to mean:
- $S$ is a subset of $T$
in the sense that $S$ is permitted to equal $T$.
That is, for which we have specified as $S \subseteq T$.
Although many sources use this interpretation, it is emphatically not recommended, as it can be the cause of considerable confusion.