User defined key going to straight to a program

[matalog]

I suppose It may be obvious, when you consider that the whole program was defined in the first line as KEY K_Sin(), effectively making it a Key reading program, not a function or procedure, but then KEY, as a command is not defined anywhere in any of the lists that I have looked at, it was merely mentioned in page 616 of the 2017 version of the Manual that I read.

Maybe it is not defined, because it is not usable anywhere else in a program?

Any advice on the subject would be appreciated.