Variants provide a flexible means of creating and maintaining variations of a single object.
Variants are objects that have been adapted based on specific criteria. Variants are always linked to a primary object, which are what you place inside books or topics. Primary objects can have an unlimited number of variants, each with their own set of variant criteria. When you filter or publish a book, Author checks if any of the objects in the book have variants. It then checks if the criteria you selected meet the criteria required by those variants. If the criteria requirements are met, Author replaces the primary object with the variant object.
Variants make it easier to maintain multiple adaptations of your content, and to assemble or reassemble existing content. For example, your topics could have variants for different countries. If you need a guide for a specific country, you'd publish it as normal, then select the required country as a variant criteria. Author-it will publish the topics that have a variant criteria matching the country you selected. The same principle can be applied to year, product, platform, or any other adaptations you require.