"Rep" was a colloquial term for representatives of the Enemy. It was used by Time Lord soldiers during the War in Heaven. (PROSE: The Taking of Planet 5)
According to the recollections of Patient Y, the Order of the White Peacock was made up of fictional entities and real people who acted as their "Reps". (PROSE: Pre-narrative Briefings)
Amara attempted to become an enemy "rep" by inhabiting the body of Mullion. (PROSE: What Keeps Their Lines Alive)
The Eighth Doctor, Fitz Kreiner, Compassion speculated on the exact meaning of the term "Rep" while believing it to refer directly to the Enemy; they joked the Enemy could be repo-men, replicas (related to shapeshifters such as Zygons, the Rutan horde, or Nestene), "Revolting Extradimensional Parasites", or reptiles (as in, 87th century Earth Reptiles with "transforming T.Rex time machines" whose causal nexus was contradicted by established human history). (PROSE: The Taking of Planet 5)