RowNumber Function (Reporting Services)
Returns a running count of all rows in the specified scope.
(String) The name of a dataset, grouping, or data region that contains the report items to which to apply the aggregate function. If a dataset is specified, the running value is not reset throughout the entire dataset. If a grouping is specified, the running value is reset when the group expression changes. If a data region is specified, the running value is reset for each new instance of the data region. For more information about the Scope parameter, see Using Report Functions in Expressions (Reporting Services)
The following code example provides a running count of rows in the outermost data region.