The criteria / parameter are clear if you have ever worked with them. I had to work with them last week the first time. The interesting part is that Apple iWorks' Numbers' documentation is very good in that part. See
"For each of the test and condition value pairs, the corresponding (same position within range or array) cell or value is compared to the condition. If all of the conditions are met, the corresponding cell or value in sum-values is included in the sum.
All arrays must be of the same size."
I think we really should improve the documentation on that part as I think that Apple did a good job in describing what the function does.
"If the range of values for calculating the sum and any range for finding criterion have unequal sizes, the function returns err:502."
from this logically follows that the range should be equal, and each element of the sequence is compared with the corresponding. In addition, I specially made an excessive number of examples with explanations of why such a result is obtained.
But, if you have knowledge how you can improve this article, without making it excessive or illogical, I think you can try it.
I propose a warning paragraph of its own just below the paragraph that start with Criterion_2, with the following content:
<emph>Sum_range</emph> and <emph>Criterion_range1, Criterion_range2...</emph> must have the same size or the function returns err:502 - Invalid argument
Olivier Hallot committed a patch related to this issue.
It has been pushed to "master":
tdf#97824 improve description of SUMIFS conditions