A Synchro Substitute Monster is a monster with an effect that allows it to replace a single Synchro Material Monster that must have a specific name or be of a specific archetype for the Synchro Summon of a Synchro Monster. Unlike a Fusion Substitute Monster, the other Synchro Material Monster(s) do not have to be the correct one(s), but there isn't a situation where this would be applicable.