property Column.GroupByFormatCell as String
Indicates the format of the cell to be displayed when the column gets grouped by.

String A String expression that may specify HTML format, <caption> and value keywords as explained bellow.
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, 

For instance:

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:

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