What is allowed as an id argument in register_sidebar( $args )

I want to register a sidebar but I am a little confused about the uses of the id argument in register_sidebar function.

Codex says: id – Sidebar id – Must be all in lowercase, with no spaces
(default is a numeric auto-incremented ID).

Of what use is the id argument and must it always be in numeric form?.

The sidebar ID is used to uniquely identify this specific sidebar. If you don’t set it and something creates another, you could find that your sidebar moves somewhere unexpected!

It doesn’t need to be numeric – you can use strings, too.