**Bug introduced in 10.2.0 or earlier and fixed in 10.4.0** ---------- Consider the following code. ClearAll["Global`*"] InactSum = Inactive[Sum] InactInt = Inactive[Integrate] A = InactInt[ Subscript[\[Phi], j][x] InactSum[ Subscript[a, i] Subscript[\[Phi], i][x], {i, 1, n}] , {x, a, b}] B = InactInt[ InactSum[Subscript[a, i] Subscript[\[Phi], i][x] Subscript[\[Phi], j][x], {i, 1, n}] , {x, a, b}] Interchange = InactInt[InactSum[p_, q_], r_] -> InactSum[InactInt[p, r], q]; A /. Interchange B /. Interchange I have introduced two expressions $$\begin{align} A &= \int_{a}^{b} \phi_{j}(x) \left[ \sum_{i=1}^{n} a_i \phi_{i}(x) \right] dx \\ B &= \int_{a}^{b} \left[ \sum_{i=1}^{n} a_i \phi_{i}(x) \phi_{j}(x)\right] dx \end{align}$$ where in $A$, $\phi_{j}$ is outside the summation while in $B$ it is inside the summation. Technically, these are both the same. But when I was trying to write a transformation rule that interchanges the integration and summation, then I noticed that it just applies to $B$ and **not** to $A$. So, I understood that Mathematica understands the **little** difference between these two but **The Standard Output for Both is the Same!** [![enter image description here][1]][1] This prevents user from understanding the difference and it just shows itself when you want to apply the transformation rule. I think this is a bug. In my opinion, the standard output for these two should be different. I would be happy to see what you think. [1]: https://i.sstatic.net/XGR0g.png