What does $B_n(X) + S_n(A)$ mean in the definition of a relative $n$-boundary mod $A$?

Question

The group of relative $n$-boundaries mod $A$ is defined as:

$$B_n(X,A) = \{ \gamma \in S_n(X) : \gamma - \gamma' \in B_n(X) \text{ for some } \gamma' \in S_n(A)\}$$ $$= B_n(X) + S_n(A)$$

What does the addition sign mean for $B_n(X) + S_n(A)$? Is it a direct sum (indirect product) of the two subgroups of $S_n(X)$?