Variant objects are used by Author-it while you are working in the library and when you publish.
Preview variant content in the library while authoring
While working with a book it is good proofing practice to filter the book to view a representation of the variant content that you'll be publishing. Filtering the book prior to publishing gives a good indication of the state of the content. Where variants do not exist, based on the structure in the variant criteria, filtering shows the fall-back content that will be used instead.
You can also compare two variants side-by-side to quickly see the difference between them.
Publishing with variants
Use variant criteria values during publishing to select the variation of the content to publish for that job. Publishing uses translated content and, in many cases, localized object templates. Like book filtering, if variants do not exist, the filtering for publishing defines the content and objects that will be used to create the output. Localized templates provide many of the special settings and options that are needed by your publishing jobs.
See also: Using Variants with the Publishing Process.
Language pairs are specially configured for localization jobs
The language pairs in Translations are separate from the variant criteria values used in the library for filtering. The language pairs are configured in Translations Settings > Service Providers. They describe the source and target languages for a translation job. For example, creating a localization job for translating source content from US English to Canadian French.