A Grapple Voltage Door is a purple-colored door that is mentioned in the Map key in Metroid Prime 3: Corruption. Despite the inclusion of this door in the key, there are no such doors encountered in the game. Instead of being used to open doors, the Grapple Voltage is used only to power or drain terminals to open a door, or to disable shields blocking them.
It is possible Grapple Voltage Doors were intended, but scrapped, and the Map key item was left behind. Doors blocked by shields generated with the mentioned terminals are not marked on the map, similarly to Locking mechanisms (which appear far more often in Corruption than Multi-Lock Blast Shields).