thank you for the model showing the two approaches to summary items.
I first encountered the issue with summary items when I linked categories between different matrices. “Automatic” summary items will be calculated in both matrices, effectively meaning the formulas are linked (i.e. copied) as well. “Manual” summary items (i.e. the approach you are demonstrating in your model) will not be linked (copied).
There is a a clear advantage having “automatic” summary items being linked (copied) together with linked categories between different matrices (especially in large models). However if in a particular matrix you for example want absolute and relative figures side by side, a sum(summary) item will give strange results unless you can override the priority.
Maybe it would be a good thing to have the option to overrule absolute priority in “automatic” summary items.