Meaning of $\mathbb K\in\{\mathbb R,\mathbb C\}$? What is the meaning of $\mathbb K\in\{\mathbb R,\mathbb C\}$ in the following?

Let $A$ be a non-empty open subset of $\mathbb K\in\{\mathbb R,\mathbb C\}$ and let $f:A\rightarrow \mathbb K$ be a continuous function.

Is $\mathbb K$ a set of real numbers and the complex numbers?

It means that $\Bbb K$ is $\Bbb R$ or $\Bbb C$.
