In the Matrix report, the syntax for this function is PARENTGROUPVAL(summary_field, parent_row_grouping, parent_column_grouping).
The same is true with the Summary report; the PARENTGROUPVAL() function is not applicable when you select the formula displayed as All summary levels. This function is applicable only when you select At a specific row/column grouping level....
The grouping level parameter in the Summary report is split into two parameters in the Matrix report: parent_row_grouping and parent_column_grouping.
Let's create a new Opportunity report with the Matrix format, filter the report by Close Date of 3 months, and then group by Calendar Month (column) and Stage (row). Refer ...
