Synchro Substitute Monster
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.
Treated as alternate name
Some monsters can replace specifically named Synchro Materials by changing their name to that of a specifically named Synchro Material.
For example, "Pain Painter" has its name treated as "Plaguespreader Zombie" while it is face-up on the field. It can be used as the Tuner monster in the Synchro Summon of monsters such as "Archfiend Zombie Skull" that require specific Tuner monsters.
If a Tuner monster gains the name of a required non-Tuner monster for a Synchro Summon, it may be used as that monster, in which case the Synchro Summon will use multiple Tuners. If a non-Tuner monster gains the name of a required Tuner monster for a Synchro Summon, it may be used as that monster, but the Synchro Summon cannot be performed unless at least one of the Synchro Materials is a Tuner monster.
For example, "Scanner" has its name treated as "Majestic Dragon" while it is face-up on the field. Since a Synchro Summon cannot be performed without using a Tuner monster as Synchro Material, it can only be used as "Majestic Dragon" for the Synchro Summon of "Majestic Star Dragon" if "Stardust Dragon" is treated as a Tuner monster.