Syntax

SumAλ( Array1, [Array2], [ByCol] )

This is a range aware version of SUM(). When placed in the same row as a range's top left cell, this processes the range by row. When placed in the same column, this processes the range by column. 

Arguments

Array1

(Required) A range or array to apply SUM to by column or by row (depending on placement).

Array2

(Optional) A single character or an array to add to Array1.

ByCol

(Optional) When used over a range of cells, this will be FALSE if this function is placed in the same row as the array. Otherwise, this defaults to TRUE. When using this function over an array that is not associated with a range, this will default to TRUE.