This feature adds more sorting and grouping options to the normal Text & Date sorting already available.
After tapping 
and then "Custom..." you can specify exactly how you want to sort and group your data.
Sorting & grouping
Sorting can be by Text, Date or Keyword.
The new sorting by Keyword uses any text of the form "Keyword: value" in your items. It looks for the "Keyword:", then sorts by the value.
Grouping, if enabled, can be done by Date, Keyword or Tag.
Grouping by keyword works just like sorting. It looks for texts like "Keyword: value", then groups all items with the same value together under one heading.
"Tag" grouping looks for any tags on an item, then groups by the tags you have set here.
"Tag" grouping also looks for tags two levels up. E.g. if you have tagged items "High" and "Low" with "Priority", then grouping by Priority will show any items that are tagged with "High" or "Low" under this heading. Try it!