P4X ("Prison for Xev") is a reference to the P4W (Prison for Women) in Kingston, Ontario, Canada which was the only women's maximum security prison in Canada before it closed in 1997.
Guest actors Craig Charles and Hattie Hayridge are well known for playing Dave Lister and Holly, respectively, in the long running British science fiction sitcom Red Dwarf (1988).
Though Kai and Zev's origin planets have been known since early in the series (Brunnis-2 and B3K, respectively), Stanley's home planet of Ostral B is revealed for the first time.