Tree Data is typically used when the data to be displayed is 'hierarchical' e.g. a Company Organisation chart or a list Folder / File contents.
Unlike Master / Detail views (where you typically have a different grid when you expand to see children), Tree Data views show the same set of columns when a row is expanded.
Tree Data differs to normal Row Grouping because the top row in each 'Group' is a 'real' record and not merely aggregated data (though it can show agg data as in the 'Staff' and 'Budget' cols here.)
AdapTable works normally when using Tree Data as this example demonstrates - it lists the UK Government Cabinet - each 'top row' is the Cabinet member and expanded rows are Ministers.
We have (completely randomly) added fictitous 'Staff' and 'Budget' cols. These both tick more than 1,000 times a second - purely for demo purposes.
See more about Tree Data: here including some of the restrictions it includes (e.g. no row grouping or pivoting is possible).