In QGIS 3.4 report composer, when making a field group section, there is a selection under 'layer' labelled 'field'. Here, one seems to be able to select from all the fields in the layer. But as far as I can tell, this selection doesn't actually do anything. The report produces one map zoomed to each feature in the layer regardless of what field is selected here. Tables filtered by atlas id still produce one result per feature and there's no @layout_id variable. I can't fathom what the 'field' option is doing here.
So I just tested this with the states of Australia (ordered by name) to make sure I was correct.
There are three different outcomes you can achieve with this feature:
- Don't use it: the pages will be ordered in the same order as the original data table / dbf file.
- Select a field: the pages will be ordered by that field in descending order.
- Select a field and specify "ascending order" in the checkbox below: I'll let you figure that one out.
That's all there is to it.