Typically a book can only be labeled as a bestseller when it has appeared on a published bestseller list. Books can be labeled as a "National Bestseller" once they've appeared on a bestseller list published in a national publication, like The National Post. An "International Bestseller" appears on lists across at least two countries. Books can claim "#1 bestseller" status if, you guessed it, they appeared in the #1 spot. Sometimes the list is credited, for example, "A Globe & Mail bestseller." Other times a book might be credited as being written by a "Bestselling author," which means one or more of the author's previous books made its way onto a recognized bestseller list.
Once a book has made it onto a published bestseller list, it's up to the publisher to decide how to communicate that information in the book's promotional materials. A good practice is to use the Promotional Headline to call out the fact that the book is a bestseller. Publishers can also use the Cited Content fields in ONIX 3.0 to record that the book was on a bestseller list, the name of the publication or list, its position on the list, among other details. Using those fields will allow for future searchability when ONIX 3.0 becomes widely adopted.
