It's not quite as intuitive as it could be but it's easy enough once you work it out. Historical sites are shown as blue spots which you can select (this is the bit that could be more intuitive) and see the information RCAHMS has on the site. Very handy for identifying features you see on your travels.
it's a secret.
Somewhere at home, Dad has a set of maps he used when in the civil defence, all marked as restricted.
Shame those aren't on the web, they showed military installations around the Spalding area of Lincolnshire