Yeah, that’s confusing!
The order they are referring is the linear order the operands are written in, as in
a + b + c.
That is, in
(a + b) + c = a + (b + c)
both sides of the equation have a then b then c. It is not the order you would do the calculations in to arrive at the answer (which does depend on grouping).
I like to think of order as “if I gave this work to different people, does it matter what order I get the answers back in? or can I just combine them as I get them?” For instance, if I had 20 people merging hashmaps, and some people are faster and some slower, I do care what order I get them back in. I would probably want them numbered to be able to reorder them.
However, I don’t care about the grouping. I could give 3 hashmaps to merge to the first person, 2 to the second, 15 to the third, etc. Or I could give 1 to the first person, 20 to the second person, etc. As long as the order is maintained, I can group them arbitrarily.
I hope this helps! I think you’ve got it, though.