To generate a reference to the "last n columns" in a table, we build a reference in two parts, joined by the range operator. To get a reference to the left column, we use:
Since data contains 6 columns, and K4 contains 3, this simplifies to:
INDEX(data,0,4) // all of column 4
To get a reference to the right column in the range, we use:
Which returns a reference to column 6 of the named range "data", since the COLUMN function returns 6:
INDEX(data,0,6) // all of column 6
Together, the two INDEX functions return a reference to columns 4 through 6 in the data (i.e. F5:H8), which resolve to an array of values inside the SUM function:
The SUM function then calculates and returns the sum, 135.