This is the ID that represents the publication version/variant. It must be unique within the locale (loc_id). You will see this appears in the URL. It can be any set of alpha numeric characters. This becomes more relevant in the future when you can create and publish multiple variants of a book and switch between them similar to languages.
This is a description/name of the version/variant. It is used for internal reference and is not displayed in the application.
This must point to the sub-folder under content folder where the localized topics and images/media are stored for the locale. This can be any set of alpha numeric characters, but recommend you stick to a code that makes is easy to identify the locale uniquely so en-uk, en-ca, and en-gb are appropriate examples.