Every National Rail station has a specific 3-character station code associated with it. For example, for London Waterloo the code is WAT; for Newcastle it’s NCL.
Because these codes are for specific stations only, you won’t see them for groups of stations. For example, “Birmingham (all stations)” does not have a 3-character station code.
These station codes are used throughout the app in various places, and you can use them to search for stations on the “Choose station” screen.