SUM(...)

Category: Math and trigonometry

Adds its arguments

SUM(number1,[number2],...)

Category: Lookup and reference

Returns a reference offset from a given reference

OFFSET(reference, rows, cols, [height], [width])

Category: Lookup and reference

Looks up values in a reference or array

MATCH(lookup_value, lookup_array, [match_type])

0

The formula relies on OFFSET() function because multiple values can not be returned by VLOOKUP or more accurately, VLOOKUP is unable to return range object. The thus returned values from the range is summed by SUM() formula.