A potential engine was a device used by the Celestial Intervention Agency to read individuals' timelines, which would enable the user to ensure their chosen outcome of the timeline came to pass. They were used by agents in the field.
Following her retirement from the Agency, Maris was allowed to keep a potential engine as a gift. She attempted to use it on the First Doctor as part of her investigation into where he had gone after stealing a TARDIS, but the engine produced hundreds of contradictory records. (PROSE: Celestial Intervention - A Gallifreyan Noir)