Minecraft wiki entry for stats, associated text: selector (set mode only)
Specifies the selector to be evaluated when a command is run by the specified block or entity to determine which entity(ies) scoreboard objective is to be updated with the result returned by stat. May be anything because it is not evaluated until a command is run by the specified block or entity, but only a player name or a target selector produces useful results (though player names can be fake, so even real players don't need to be online).