Does anyone know what this symbol means? Where would one use it? Someone recently asked me but I do not know what it means. I have seen it with just one line underneath to denote subset. With an equals sign would it mean the trivial subset, meaning $A$ is a subset of $A$?
On
Saying $A\subset B$ or $A\subseteq B$ means that $A$ is a subset of $B$. Some people use $A\subset B$ meaning that $A\subseteq B$ with $A\neq B$. Another notation for this is $A\subsetneq B$.
Writing $B \supset A$, $B\supseteq A$ and $B\supsetneq A$ is equivalent to the above discussion. There is no difference between $\subseteq$ and $\subseteqq$, etc.
It means "is a superset of" as in $\{1,2\}\supseteq\{1\}$.