Subtotal
The Subtotal function returns the subtotal for an aggregate formula.
Syntax
Subtotal(aggregate, mode, [parameters])
Function arguments:
 aggregate (required)  The aggregate formula to use

mode (required)  The mode to use when calculating the aggregate formula. Mode determines which dimension(s) to use when performing the calculation.
 General purpose modes

"grand_total"
(default for visualizations and pivot tables)  Calculates the subtotal for the aggregate formula.

 Visualization modes

"color"
 Calculates the aggregate formula using only the column specified for COLOR. 
"x_axis"
 Calculates the aggregate formula using only the column(s) placed on the XAXIS. 
"trellis_column"
/"trellis_row"
 Calculates the aggregate subtotal using only the dimension specified for the TRELLIS COLUMN or TRELLIS ROW.

 Pivot table modes

"column"
/"row"
 Calculates aggregate subtotal for the column or row total. 
"column_parent"
/"row_parent"
 Calculates the aggregate subtotal for the column or row total, relative to the subtotal calculation for the parent dimensions.

 Table modes

"parent_grouping"
 Calculates the aggregate formula using the grouping keys from a parent grouping. This is equivalent to creating the aggregate calculation in a parent grouping and referencing it from a column in a lower grouping.

 General purpose modes

parameters (optional)  An additional parameter specific to the mode.
 This is only applicable to the following modes: "column_parent", "row_parent" and "parent_grouping". For these modes, this 3rd parameter specifies how many parent dimensions to ignore.
Examples
Subtotal(Avg([Price]), "column")
 Calculates the aggregate subtotal for each column of the Product Type dimension.

Verify that the calculation values match the builtin pivot subtotals at the bottom of the table.
Subtotal(CountDistinct([Product Type]), "x_axis")
 Calculates the aggregate subtotal for each Product Type, across all values of Store Region.

The calculation is shown in the tooltip, but is usable anywhere an aggregate formula is allowed.