The code typically refers to the fifth episode of an eighteenth season. The most prominent show with this specific numbering is the legendary motoring program Top Gear . Top Gear (UK) - Series 18, Episode 5
Jeremy Clarkson and James May explore the history of Saab, highlighting its engineering eccentricities, such as the ignition being located between the seats and the company's roots in aviation. [S18E5] Episode #18.5
Episode 18.5, titled "All of This," is a recap episode that summarizes the events of the series up to that point. Community consensus on Reddit generally suggests it is skippable if you are binge-watching. The code typically refers to the fifth episode
Richard Hammond conducts a spectacular race in the Alps, pitting a rally-spec Skoda Fabia S2000 against Swiss stuntman Yves Rossy, who uses a rocket-powered wing to fly through the mountains. Episode 18
If you are looking for a show specifically using the notation (often used for mid-season recaps or specials), it likely refers to one of the following: