PARENTGROUPVAL is a function used in custom summary formulas for reports that allows you to calculate values relative to a parent grouping or to the report grand total.
Literally, PARENT GROUP VAL stands for Value Grouping Parent..
Each report grouping level is highlighted in blue. The different shades of blue allow us to identify the parent grouping level: we can only compare the value of a grouping with a grouping of darker blue.
Example: In a report we want to display the percent share of each business sector versus the total annual revenue account:
Sum of a sector revenue account/(grouping)
Total sum of all revenue accounts (report Grand Total)