By default, the GroupByTotalField property is "count(current,rec,1)", which count recursively leaf items ( implies recursively leaf items ) of the grouping item. At runtime, the computed value of this formula is replaced in the HTML format being specified by the GroupByFormatCell property, for the value keyword. When the control is performing a group-by operation, the Items.CellValue(Item,Items.GroupItem(Item)) property is set on GroupByTotalField property, and the Items.CellValueFormat(Item,Items.GroupItem(Item)) is exHTML + exTotalField (5), where the Item is the handle of the item being added during grouping or the Item parameter of the AddGroupItem event. The GroupByTotalField property determines the formula to be applied to the column when it gets grouped. The GroupByFormatCell property determines the format of the cell to be displayed in the grouping item, when the column gets sorted.

For instance 

• "count(current,dir,1)" counts the number of child items ( not implies recursively child items ).
• "count(current,all,1)" counts the number of all child items ( implies recursively child items ).
• "sum(parent,dir,%1=0?0:1)" counts the not-zero values in the second column (%1)
• "sum(parent,dir,%1 + %2)" indicates the sum of all cells in the second (%1) and third (%2) column that are directly descendent from the parent item. 
• "sum(all,rec,%1 + %2)" sums all leaf cells in the second (%1) and third (%2) columns.

The syntax for the GroupByTotalField property property should be: aggregate(list,direction,formula) where:

aggregate must be one of the following: 

• sum - calculates the sum of values.
• min - retrieves the minimum value.
• max - retrieves the maximum value.
• count - counts the number of items.
• avg - calculates the average of values.

list must be one of the following:

• a long expression that specifies the index of the item being referred.
• a predefined string expression as follows:
  • all - indicates all items, so the formula is being applied to all items. The direction has no effect.
  • current - the current item.
  • parent - the parent item.
  • root - the root item.

direction must be one of the following:

• dir - collects the direct descendents. 
• rec - collects the leaf descendents ( leaf items ). A leaf item is an item with no child items.
• all - collects all descendents.

Currently, the following items are excluded by aggregate functions:

• not-sortable items. The SortableItem property specifies whether the item can be sorted ( a sortable item can change its position after sorting, while a not-sortable item keeps its position after sorting. 
• not-selectable items. The SelectableItem property specifies whether the user can selects/focus the specified item.
• divider items. The ItemDivider property specifies whether the item displays a single cell, instead displaying whole cells.

In conclusion, aggregate functions counts ONLY items that are:

• sortable, SortableItem is True, by default.
• selectable, SelectableItem is True, by default.
• not divider, ItemDivider is -1, by default.

Shortly, by setting to a different value to any of these properties, makes the item to be ignored by the aggregate functions.

For instance 

• count(current,dir,1) counts the number of child items ( not implies recursively child items ).
• count(current,all,1) counts the number of all child items ( implies recursively child items ).
• count(current,rec,1) counts the number of leaf items ( implies recursively leaf items ).
• count(current,rec,1) counts the number of leaf items ( a leaf item is an item with no child items ).
• sum(parent,dir,%1=0?0:1) counts the not-zero values in the second column (%1)
• sum(parent,dir,%1 + %2) indicates the sum of all cells in the second (%1) and third (%2) column that are directly descendent from the parent item. 
• sum(all,rec,%1 + %2) sums all leaf cells in the second (%1) and third (%2) columns.