Adding ArcGIS Online Tables as a data source
Besides connecting CSV files to your report, the button also allows you to connect a Table element. Within ArcGIS Online you can publish a CSV file as a Table which enables you to apply certain filters or to specify the data type of each column. This can be useful e.g. if you wish to display categoric values left aligned in a Report Builder table widget. Data values added through a CSV file will always be treated as numeric and therefore appear right aligned in a table widget.
Tables have a further advantage in that if your csv is very large then it can become slow. The whole csv file has to be downloaded to do anything with it. Whereas if the csv is converted into a table then it is queried by ArcGIS Online and you only pull back the data that is being used by particular widgets.
When adding an ArcGIS Online Table to your report data it is important that the Table has a column named the same as the Feature ID field from your feature service. This column needs to contain matching feature codes.