In a recent assignment, my professor used the following notation which is giving me a bit of trouble. I hope someone can clarify. Suppose $R$ is an integral domain and $K :=\text{Frac}(R)$, the field of fractions of $R$. He writes,
$$ "\text{Let } M \subseteq K \text{ be a module over } R"$$
Of course, $K$ is a field and $M$ is an $R$-module, but it's difficult for me to see how, as different algebraic objects, one can be a subset of another. I'm not very familiar with modules yet, so I may be missing something obvious. Could someone clarify? Thanks!