Earlier quoted context omitted.
The article says: > Sfyria, […] is not technically a language; linguists refer to it as a speech registrar, like shouting or whispering. I've never heard the term registrar (in the linguistic sense), and Google doesn't find anything relevant either, so I wonder if this is the correct one.
I think they meant "register" which is a valid linguistic term applied to this kind of thing. eg. https://books.google.co.uk/books?id=XzzABgAAQBAJ&pg=PA1&lpg=... specifically calls whistled speech a speech register.
Yep. Register is basically a specific version of a language used on certain cases. Like English has a formal register you'd use with your boss and in an academic paper. But you ain't gonna be using it often with friends, y'know?