If you have many abstract categories you can now split them in abstract categories, from which the abstract submitter can choose. After choosing a category a popup will apppear which shows the sub categories.
Here an example from the abstract submission without main categories.
You can create main categories after adding the abstract categories in Abstract - Abstract Setup - Categories
Just write in the dropdown "Group" to create a main category.
Here an example of how it looks in the abstract submission afterwards
A mix up of both is not working, when you use abstract categories, then categories without a main category will not be shown in the abstract submission.