It's not about "other" having a meaning unto itself; rather it's about having a placeholder in the enum "space" (for older languages that implement it as an int

It's not about "other" having a meaning unto itself; rather it's about having a placeholder in the enum "space" (for older languages that implement it as an integer) so that customers can introduce their own variants. My library would then happily accept any value passed in, but only act upon the enum values that the library itself specifies. The customer would be entirely responsible for making sure their own variants behave well. This is not a common thing to do, of course, but when your customers are clamoring for what would actually be a useful feature for your product, even this somewhat ugly hack is a lot better than saying "no".