By default, the GroupByFormatCell property is "'<b><caption></b> (' + value + ')'", which indicates that the grouping label is shown in bold, followed by the computed value of the GroupByTotalField property. The GroupByFormatCell property determines the format of the cell to be displayed in the grouping item, when the column gets sorted. The GroupByTotalField property determines the formula to be applied to the column when it gets grouped. When the control is performing a group-by operation, the Items.FormatCell(Item,Items.GroupItem(Item)) property is set on GroupByFormatCell property, where the Item is the handle of the item being added during grouping or the Item parameter of the AddGroupItem event. 

In conclusion, 

• the <caption> keyword in the GroupByFormatCell property is replaced with the grouping label/value, and the result expression is passed to the FormatCell property. 
• the value keyword indicates the computed value of the GroupByTotalField property.

For instance:

• the "'<b><caption></b> (' + currency(value) + ')'" displays the grouping label, and the aggregate field as a currency, as specified in the regional settings.
• the "'<b><caption></b> (' + currency(value) + `, inc. VAT ` + currency(1.19*value) + ')'" displays the grouping label, and the aggregate field, including a computed field ( VAT ) as a currency, as specified in the regional settings.
• the "'<b><caption></b> <fgcolor=808080><font ;6>(Total ' + (value format ``) + ')</font></fgcolor>'" displays the grouping label, and the aggregate field as a current number format, as specified in the regional settings, with a different font and foreground color.

The value keyword in the GroupByFormatCell property indicates the value to be formatted ( as a result of the GroupByTotalField property ):

The expression supports cell's identifiers as follows:

• %0, %1, %2, ... specifies the value of the cell in the column with the index 0, 1 2, ... The CellValue property specifies the cell's value. For instance, "%0 format ``" formats the value on the cell with the index 0, using current regional setting, while "int(%1)" converts the value of the column with the index 1, to integer.
• %C0, %C1, %C2, ... specifies the caption of the cell, or the string the cell displays in the column with the index 0, 1 2, ... The CellCaption property specifies the cell's caption. The cell's value may be different than what the cell displays as a string. For instance, let's say a cell display HTML format. The %0 returns the html format including the HTML tags, while %C0 returns the cell's content as string without HTML tags. For instance, "upper(%C1)" converts the caption of the cell with the index 1, to upper case, while "%C0 left 2" returns the leftmost two characters on the cell with the index 0.
• %CD0, %CD1, %CD2, ... specifies the cell's extra data in the column with the index 0, 1 2, ... The CellData property associates any extra/user data to a cell. For instance, "%CD0 = `your user data`" specifies all cells whose CellData property is `your user data`, on the column with the index 0.
• %CS0, %CS1, %CS2, ... specifies the cell's state in the column with the index 0, 1 2, ... The CellState property specifies the cell's state, and so it indicates whether the cell is checked or un-checked. For instance, "%CS0" defines all checked items on the column with the index 0, or "not %CS1" defines all un-checked items in the column with the index 1.

This property/method supports predefined constants and operators/functions as described here.