This tag is used on railway stations (railway=station) and some other places (for example, bus stations and ferry ports with rail through ticketing) within Britain on the National Railway network to indicate the railway station code. It is a public facing code which allows customers to check train times & plan journeys via websites.
There are some railway stations which have distinct sub-stations (such as a separate high-level and a low-level station). In this case, the separate sub-stations may have an internal CRS code (called the "CRS reference code" in the official documentation). This should also be put in the ref:crs=* tag as well. Examples include London Paddington (ref:crs=PAD) and Paddington Crossrail (ref:crs=PDX), although the latter isn't used in journey planning and ticketing.
Examples
See also