what is meaning $ A(O_K)$?

Question

I was reading the paper " The number of extensions of a number field with fixed degree and bounded discriminant." by Jordan S. Ellenberg and Akshay Venkatesh(https://arxiv.org/pdf/math/0309153.pdf). In the second section pg no 726 they define $ A=Spec(R)$ Later use some notation $A(O_K)$. need some help with understanding notation.

Answer 1

As suggested, I am posting my comment as an answer:

It seems like scheme-theoretic notation. You can see it as the scheme morphisms $Spec(O_K)→A$, or, better, as the morphisms of $O_K$-algebras $R \rightarrow O_K$.