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, ...{any} specifies the value of the cell in the column with the index 0, 1 2, ... The CellValue property defines the cell's value. For example, "%0 format ``" formats the value in the cell at index 0 using the current regional settings, while "int(%1)" converts the value in the cell at index 1 to an integer.
• %C0, %C1, %C2, ...{string} specifies the caption of the cell, or the string the cell displays in the column with the index 0, 1 2, ... The CellCaption property gets the cell's formatted caption. The cell's displayed string may differ from its actual value. For example, if a cell displays HTML content, %0 returns the HTML format including the tags, while %C0 returns the cell's content as a plain string without HTML tags. For instance, "upper(%C1)" converts the caption of the cell at index 1 to uppercase, while "%C0 left 2" returns the leftmost two characters of the caption in the cell at index 0.
• %CD0, %CD1, %CD2, ...{any} 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 example, "%CD0 = your user data" specifies all cells in the column with index 0 whose CellData property is equal to your user data.
• %CS0, %CS1, %CS2, ...{number} specifies the cell's state in the column with the index 0, 1 2, ... The CellState property defines the state of a cell, indicating whether it is checked or unchecked. For example, "%CS0" identifies all checked items in the column with index 0, while "not %CS1" identifies all unchecked items in the column with index 1.
• %CT0, %CT1, %CT2, ... {boolean} returns true if the cell displays a total field; otherwise, it returns false. The exTotalField / exTotalColumn flag specifies whether the cell displays a total field. For instance, "%CT1" refers to all cells in the second column that display totals, while "not %CT1" refers to all cells in the second column that do not display totals.
• %CE0, %CE1, %CE2, ... {boolean} returns true if the cell is editable; otherwise, it returns false.. For example, "%CE0" refers to all editable cells in the first column, while "not %CE1" refers to all cells in the second column that are read-only.
• %CC0, %CC1, %CC2, ... {number} retrieve the number of child items (this keyword consistently returns identical results for all cells since it pertains to the item that hosts each cell). The ChildCount property returns the number of child items. For example, "%CC0" identifies all parent items, while "%CC0 = 0" identifies all leaf items.
• %CX0, %CX1, %CX2, ... {boolean} returns true if the item hosting the cell is expanded, or false if it is collapsed (this keyword consistently returns identical results for all cells since it pertains to the item that hosts each cell). The ExpandItem property specifically indicates whether the item is expanded or collapsed. For example, "%CX0" refers to all expanded items, while "not %CX0" identifies all collapsed items

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