Spreads move together in tandem regardless of the number of legs it has. In this spreadsheet, there are none over two (2) legs but a longer export will have more legs. A row will have the same Pos# if any of legs of the Spread have matching I/S#'s. They move up under their counterpart (in tandem) based on whether the position numbers are the same.
The Inst# column also should be renamed to I/S#.