The Help text says: 4. Highest Period Amount: This method calculates fees based on the highest ending balance among the selected periods.
This method should only be selected for quarterly or annual fee frequencies. You can select it for monthly, but it is the same calculation as Ending YTD balances since there is only one period in a monthly calculation.
In a quarterly fee calculation, if January ends with $0, February with $1000 and March with $500, The fee will calculate on a $1000 basis. If you had chosen YTD, if would calculate on a $500 basis.
In a monthly calculation using the same values as above, the January fee would be based on $0, February on $1000 and March on $500.
In February, if a gift came in on the 5th for $50,000 and a grant went out on the 10th for the same $50,000, the ending balance is still $1000, and the basis would be calculated on the $1000 under the YTD and Highest period amount calculation choices.
The average daily balance (adb) choice (#5) would give you the highest basis since it would calculate the adb on $1000 for 4 days, $100,000 for 5 days and $1000 for the remaining month.