The set of all the summands/factors in a sum/product
The set containing all the summands of $\sum_{k=1}^n a_k$ is given by $\{ a_k | 1\leq k \leq n\}$.
We don't speak of "ordered sets" in math. If you want something like an ordered set then you are likely looking for a tuple. As for the tuple containing all summands of $\sum_{k=1}^n a_k$, this tuple could be identified with the function $f:\{1,2,\ldots,n\}\to\mathbb{R}$ given by $f(k) = a_k$. In other words, this is just the finite sequence $(a_k)_{k=1}^n$.