This would be a footer row on list / list part pages.
When selecting multiple records on a list page it would be useful to see a summary of those selected rows. Primarily the total of certain fields (eg Amount / Quantity fields)

When working on reconciliations or interrogating the system I often need to export to Excel just to selected several rows and see what they total.

This could be specified in AL as a page field property "SummaryType" with options such as None, Sum, Average, Min or Max
Or even better, the user could chose the fields to summarise as a part of their personalisation.
Or maybe the SumIndexFields could be used to determine which values to summarise.

Taking the G/L Entries page as an example.
I would open the page and specify filters, the footer would show me the total of Amount for the filtered records.
I could then shift / ctrl click to select multiple records and see the totals for the selected rows only