Just whatever data is being shown interactively, to also be available as a file that you can download (in order that you can process it locally).
Otherwise, if you have a link to the interactive report but not to the data as a downloadable file, it won't work so well; users (at least myself) would complain.
Thanks for elaborating – I think I see your point.
The good news is, for your own GRID documents, they're going to be backed by a spreadsheet file you made or already have access to.
However, in the case of GRID documents made by others, it's up to them if they want to make the underlying spreadsheet model available to the users viewing the GRID document. You can see an example of this here: https://grid.is/@sigrid/scenario-analysis-user-funnel-P1yGUF... – at the bottom of the document, the document author has made the spreadsheet available for anyone.
Otherwise, if you have a link to the interactive report but not to the data as a downloadable file, it won't work so well; users (at least myself) would complain.