A Sum of the Parts valuation involves valuing each individual division of a company separately and then adding them together to get a total combined value.
This type of valuation analysis is generally used when valuing a company that has multiple, unrelated business divisions such as in a conglomerate like Berkshire Hathaway or General Electric.
Berkshire Hathaway, for example, owns several subsidiary businesses in completely different industries such as insurance, manufacturing, and retail. Each of these businesses have completely different comparable companies and precedent transactions that wouldn't be used to value the entire company. Instead, you would use different comp sets for each individual division or business, value them all separately, and then add them all together to get the combined value of the conglomerate.