sigma notation for sumsets $\Sigma B$

Question

I am reading a number theory paper about sumsets [1]. If $A, B \in \mathbb{Z}$ are two sets of integers we can define: $$ A + B = \{ a + b : a \in A, b \in B \} \subseteq \mathbb{Z}$$ what do you think they mean by sigma notation for subsets. The paper has the notation: $$ \sum_{i \in I} B_i \subseteq A $$ could that mean the sumset of all the sets $B_i$ with $i \in I$ ? There are no numbers to add quite yet.